Method of producing water absorbent resin

ABSTRACT

A method for producing a water absorbent resin, the method including a polymerization step, a drying step, a classification step, and a surface crosslinking step. The classification step carried out before or after the surface crosslinking step requires use of a metal sieve mesh having stretch tension from 35 to 100 N/cm, in which an air jet cleaner or an air jet brush cleaner and plural tapping balls or tapping blocks are installed below the metal sieve mesh, a classification aid particle having specific gravity different from that of the water absorbent resin powder is added, and a fine powder of the water absorbent resin and the classification aid particle are removed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2011/056423 filed on Mar. 17, 2011, which claims priority toJapanese Application No. 2010-061224 filed Mar. 17, 2010 and JapaneseApplication No. 2010-061223 filed Mar. 17, 2010. The contents of theprior applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a method for producing a waterabsorbent resin, and more particularly, to a production method forobtaining a water absorbent resin exhibiting high liquid permeabilityunder high pressure, by performing surface crosslinking.

BACKGROUND ART

A water absorbent resin (SAP/Super Absorbent Polymer) is awater-swellable and water-insoluble polymer gelling agent, and isfrequently used primarily in disposable applications as water absorbentarticles such as paper diapers and sanitary napkins, as well as waterretention agents for agricultural and horticultural use, industrialwater stopping materials and the like. As to such a water absorbentresin, many monomers and hydrophilic polymers have been proposed as rawmaterials. However, a polyacrylic acid (salt)-type water absorbent resinwhich uses acrylic acid and/or salts thereof as monomers is inparticular industrially most frequently used due to their high waterabsorption performance.

The water absorbent resin is produced through various steps such as apolymerization step, a drying step, a pulverization step, aclassification step, and a surface crosslinking step (Patent Literature1 to 3). Furthermore, along with a performance improvement and thicknessreduction of a paper diaper that is its principal application, there isa demand for an increase in use amount of water absorbent resin (forexample, 50% by weight or more), and for many physical properties(functions). Examples thereof include absorption capacity, gel strength,extractables (Patent Literature 4), absorption speed, absorptioncapacity under load (Patent Literature 5), liquid permeability (PatentLiterature 6 to 9), particle size distribution, urine resistance,antibacterial property, impact resistance, powder fluidity,deodorization property, coloration resistance, low dust property, andthe like. Among these physical properties, liquid permeability is a moreimportant factor, and there have been suggested many methods ortechnologies for improving liquid permeability under load such as SFC(Saline Flow Conductivity; Patent Literature 6) and GBP (Gel BedPermeability; Patent Literature 7 to 9) and liquid permeability withoutload.

Furthermore, in connection with the physical properties described above,many technologies combining plural parameters have also been suggested.For example, a technology for defining impact resistance (FI) (PatentLiterature 10), a technology for defining absorption speed (FSR/Vortex)or the like (Patent Literature 11), and a technology for defining theproduct of liquid diffusion performance and amount of core absorptionafter 60 minutes (DA60) (Patent Literature 12) are known.

Furthermore, as methods for enhancing liquid permeability (SFC, GBP orthe like), there are known a technology of adding gypsum beforepolymerization or during polymerization (Patent Literature 13), atechnology of adding a spacer (Patent Literature 14), a technology ofusing a nitrogen-containing polymer having a nitrogen atom of 5 to 17[mol/kg] which is capable of protonation (Patent Literature 15), atechnology of using a polyamine and a polyvalent metal ion or apolyvalent anion (Patent Literature 16), a technology of coating a waterabsorbent resin having pH of less than 6 with a polyamine (PatentLiterature 17), and a technology of using polyammonium carbonate (PatentLiterature 18).

In addition to these, a technology of using a polyamine at extractablesportion of 3% by weight or more, and a technology of defining thesuction index (WI) or gel strength (Patent Literature 19 to 21) areknown. Furthermore, in order to improve coloration and liquidpermeability, a technology of controlling methoxyphenol which is apolymerization inhibitor used at the time of polymerization and thenusing a polyvalent metal salt (Patent Literatures 22 and 23) is alsoknown.

PRIOR LITERATURE Patent Literature

-   Patent Literature 1: U.S. Pat. No. 6,727,345-   Patent Literature 2: U.S. Pat. No. 7,193,006-   Patent Literature 3: U.S. Pat. No. 6,716,894-   Patent Literature 4: U.S. Reissue Pat. No. 32649-   Patent Literature 5: U.S. Pat. No. 5,149,335-   Patent Literature 6: U.S. Pat. No. 5,562,646-   Patent Literature 7: US 2005/0256469-   Patent Literature 8: U.S. Pat. No. 7,169,843-   Patent Literature 9: U.S. Pat. No. 7,173,086-   Patent Literature 10: U.S. Pat. No. 6,414,214-   Patent Literature 11: U.S. Pat. No. 6,849,665-   Patent Literature 12: US 2008/0125533-   Patent Literature 13: US 2007/0293617-   Patent Literature 14: US 2002/0128618-   Patent Literature 15: US 2005/0245684-   Patent Literature 16: WO 2006/082197 A-   Patent Literature 17: WO 2006/074816 A-   Patent Literature 18: WO 2006/082189 A-   Patent Literature 19: WO 2008/025652 A-   Patent Literature 20: WO 2008/025656 A-   Patent Literature 21: WO 2008/025655 A-   Patent Literature 22: WO 2008/092843 A-   Patent Literature 23: WO 2008/092842 A

SUMMARY OF INVENTION

In the aforementioned Literature 1 to 23 etc., many surface crosslinkingtechnologies, additives, and modifications of production processes havebeen suggested for an enhancement of the physical properties of waterabsorbent resins.

However, modification or addition of the raw materials of waterabsorbent resins, such as a surface crosslinking agent and additives (apolyamine polymer, an inorganic fine particle, a thermoplastic polymerand the like) have caused in some cases a decrease in the safety of theraw materials, an increase in cost, as well as a decrease in otherphysical properties. Furthermore, the addition of new production stepnot only causes high capital investment or an increase in cost due tothe energy used therein, but also requires industrially complicatedoperation, so that the addition may rather cause a decrease inproductivity or physical properties. Furthermore, there has also been aproblem that in continuous production of a huge scale (particularly, 1[t/hr] or more), physical property such as liquid permeability (SFC)deteriorates with the production time. Further, inspection ofdeterioration of the physical property takes time, so that a largeamount of substandard products sometimes has been produced in theproduction of a huge scale.

Thus, in order to improve the problems described above, it is an objectof the present invention to provide a method of enhancing andstabilizing the physical property (for example, liquid permeability) ofa water absorbent resin by a simple technique, without requiring anymodification of raw materials or high capital investment.

In order to solve the problems described above, the methods forproducing a water absorbent resin of the present invention (first andsecond aspects of the invention) are as follows.

That is, a method for producing a water absorbent resin according to thepresent invention (first aspect of the invention) is a method forproducing a water absorbent resin, which includes a polymerization stepof polymerizing an aqueous solution of acrylic acid (salt) to obtain awater-containing gel-like crosslinked polymer; a drying step of dryingthe water-containing gel-like crosslinked polymer to obtain a waterabsorbent resin powder; a classification step of classifying the waterabsorbent resin powder; and a surface crosslinking step of surfacecrosslinking the water absorbent resin powder, wherein in theclassification step that is carried out before the surface crosslinkingstep and/or after the surface crosslinking step, the stretch tension(tension) of a metal sieve mesh used in the classification step is from35 [N/cm] to 100 [N/cm].

Furthermore, another method for producing a water absorbent resinaccording to the present invention (second aspect of the invention) is amethod for producing a water absorbent resin, which includes apolymerization step of polymerizing an aqueous solution of acrylic acid(salt) to obtain a water-containing gel-like crosslinked polymer; adrying step of drying the water-containing gel-like crosslinked polymerto obtain a water absorbent resin powder; a classification step ofclassifying the water absorbent resin powder; and a surface crosslinkingstep of surface crosslinking the water absorbent resin powder, whereinin the classification step that is carried out before the surfacecrosslinking step and/or after the surface crosslinking step, anairbrush is used below the metal sieve mesh used in the classificationstep.

Meanwhile, the method for producing a water absorbent resin according tothe present invention can also be referred to as a method for enhancingliquid permeability of a water absorbent resin. Furthermore, the methodfor producing a water absorbent resin according to the present inventioncan also be referred to as a method for classifying a water absorbentresin.

According to the present invention, in a method for producing a waterabsorbent resin including a polymerization step, a drying step, aclassification step, and a surface crosslinking step, the physicalproperty after surface crosslinking (for example, liquid permeability)can be enhanced, and deflection (standard deviation) of the physicalproperty during continuous production or deterioration of the physicalproperty resulting from continuous operation can be decreased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a representative flow diagram of a continuous process for awater absorbent resin.

FIG. 2 is a schematic cross-sectional diagram, viewed from a lateralside, of a sieve classifying apparatus provided with tapping balls on apunching metal disposed below a metal sieve mesh.

FIG. 3 is a schematic cross-sectional diagram, viewed from above, oftapping balls provided on a punching metal that is divided into pluralcompartments.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, the method for producing a water absorbent resin accordingto the present invention will be described in detail. However, the scopeof the present invention is not intended to be restrained by thesedescriptions, and embodiments other than the following examples can alsobe appropriately modified and carried out to the extent that the purportof the present invention is not impaired. Specifically, the presentinvention is not intended to be limited to the various exemplaryembodiments described below, and can be modified into variousembodiments within the scope illustrated by the claims. Exemplaryembodiments that can be obtained by appropriately combining thetechnical means that are respectively disclosed in different exemplaryembodiments, are also included in the technical scope of the presentinvention.

[1] DEFINITIONS OF TERMS (1-1) Water Absorbent Resin

The term “water absorbent resin” according to the present inventionmeans a water-swellable and water-insoluble polymer gelling agent.Meanwhile, the term “water-swellable” means that the CRC (absorptioncapacity without load) defined in ERT441.2-02 is essentially 5 [g/g] orhigher, preferably 10 to 100 [g/g], and still more preferably 20 to 80[g/g]. The term “water-insoluble” means that the Ext (extractables)defined in ERT470.2-02 is essentially 0% to 50% by weight, preferably 0%to 30% by weight, more preferably 0% to 20% by weight, and particularlypreferably 0% to 10% by weight.

The water absorbent resin can be appropriately designed in accordancewith the application, and there are no particular limitations. However,the water absorbent resin is preferably a hydrophilic crosslinkedpolymer obtained by crosslink-polymerizing an unsaturated monomer havinga carboxyl group. Furthermore, the water absorbent resin is not limitedto the case in which the entire amount (100% by weight) is in the formof polymer, and the water absorbent resin may contain an additive andthe like to the extent that the performance described above ismaintained. That is, in the present invention, even a water absorbentresin composition is collectively referred to as a water absorbentresin. The content of a polyacrylic acid (salt)-type water absorbentresin is preferably 70% to 99.9% by weight, more preferably 80% to 99.7%by weight, and still more preferably 90% to 99.5% by weight, relative tothe total amount. As another component other than the water absorbentresin, water is preferred from the viewpoints of absorption speed andimpact resistance of the powder (particle), and optionally, an additivethat is described below is included.

(1-2) Polyacrylic Acid (Salt)-Type Water Absorbent Resin

The term “polyacrylic acid (salt)-type water absorbent resin” accordingto the present invention refers to a water absorbent resin whichcomposed principally of acrylic acid and/or a salt thereof (hereinafter,referred to as acrylic acid (salt)) as a repeating unit.

Specifically, it refers to a polymer which contains acrylic acid (salt)essentially 50% to 100% by mole, preferably 70% to 100% by mole, morepreferably 90% to 100% by mole, and particularly preferablysubstantially 100% by mole, among all the monomers used inpolymerization (excluding the crosslinking agent). The salt as a polymeressentially includes a water-soluble salt, and preferably includes amonovalent salt, still more preferably an acrylic metal salt or anammonium salt, particularly an alkali metal salt, and further a sodiumsalt.

(1-3) EDANA and ERT

The term “EDANA” is an abbreviation for the European Disposables andNonwovens Association, and the term “ERT” is an abbreviation for amethod for analyzing a water absorbent resin (EDANA Recommended TestMethod), which is a European standard (almost international standard).

Meanwhile, in the present invention, unless otherwise specified, thephysical properties of a water absorbent resin or the like are measuredaccording to the ERT original (published Literature: revised in 2002).

(a) CRC (ERT441.2-02)

The term “CRC” is an abbreviation for Centrifuge Retention Capacity, andmeans absorption capacity without load (hereinafter, also be referred toas “absorption capacity”). Specifically, the CRC is absorption capacity(unit: [g/g]) obtained after 0.200 g of a water absorbent resin in anon-woven fabric bag is allowed to freely swell for 30 minutes in alarge excess of a 0.9 wt % aqueous solution of sodium chloride, and thenis dehydrated in a centrifuge.

(b) “AAP” (ERT442.2-02)

The term “AAP” is an abbreviation for Absorption Against Pressure, andmeans absorption capacity under load. Specifically, the AAP isabsorption capacity (unit: [g/g]) obtained after allowing 0.900 g of awater absorbent resin to swell in a 0.9 wt % aqueous solution of sodiumchloride for one hour under a load of 2.06 kPa (0.3 psi, 21 [g/cm²]).Meanwhile, in the present invention, the measurement was made bychanging the load conditions to 2.06 kPa (0.3 psi, 21 [g/cm²]) or 4.83kPa (0.7 psi, 50 [g/cm²]).

(c) “Ext” (ERT470.2-02)

The term “Ext” is an abbreviation for Extractables, and means awater-solubilized fraction (amount of water-solubilized components).Specifically, the Ext is the value (unit: wt %) obtained by stirring 1 gof a water absorbent resin in 200 g of a 0.9 wt % aqueous solution ofsodium chloride for 16 hours, and then measuring the amount of dissolvedpolymer by pH titration.

(d) “FSC” (ERT440.2-02)

The term “FSC” is an abbreviation for Free Swell Capacity, and means theratio of free swelling. Specifically, the FSC is absorption capacity(unit: [g/g]) measured after immersing 0.20 g of a water absorbent resinin 0.20 g of a 0.9 wt % aqueous solution of sodium chloride for 30minutes, without performing dehydration with a centrifuge.

(e) “Residual Monomers” (ERT410.2-02)

The term “Residual Monomers” means the amount of monomers remaining in awater absorbent resin. Specifically, the residual monomers is the value(unit: ppm) obtained by introducing 1.0 g of a water absorbent resin in200 ml of a 0.9 wt % aqueous solution of sodium chloride, stirring themixture for 2 hours, and then measuring the amount of monomers elutedinto the aqueous solution by high performance liquid chromatography.

(f) “PSD” (ERT420.2-02)

The term “PSD” is an abbreviation for Particle Size Distribution, andmeans the particle size distribution measured by sieve classification.Meanwhile, the weight average particle size (D50) and the particle sizedistribution width are measured by a method similar to “(1) AverageParticle Diameter and Distribution of Particle Diameter” described in EPNo. 0349240, p. 7, lines 25-43.

(g) Other Physical Properties of Water Absorbent Resin Defined by EDANA

“pH” (ERT400.2-02) means pH of a water absorbent resin.

“Moisture Content” (ERT430.2-02) means water content percentage of awater absorbent resin.

“Flow Rate” (ERT450.2-02) means a flow-down speed of a water absorbentresin.

“Density” (ERT460.2-02) means bulk specific gravity of a water absorbentresin.

“Respirable Particles” (ERT480.2-02) means respirable dust of a waterabsorbent resin.

“Dust” (ERT490.2-02) means dust contained in a water absorbent resin.

(1-4) Liquid Permeability

The flow of a liquid that flows between the particles of a swollen gelunder a load or without load is referred to as “liquid permeability.”Representative measurement methods for this “liquid permeability”include SFC (Saline Flow Conductivity) and GBP (Gel Bed Permeability).

The term “SFC (saline flow conductivity)” means the liquid permeabilityof a 0.69 wt % aqueous solution of sodium chloride in 0.9 g of a waterabsorbent resin under a load of 0.3 psi. This is measured according tothe SFC test method described in U.S. Pat. No. 5,669,894 A.

The term “GBP” means the liquid permeability of a 0.69 wt %physiological saline in a water absorbent resin under a load or underfree swelling. This is measured according to the GBP test methoddescribed in WO 2005/016393.

(1-5) Others

In the present specification, the expression “X to Y” that indicates arange means “equal to or more than X and equal to or less than Y.”Furthermore, the unit of weight, “t (ton)”, means “metric ton”, andunless otherwise specified, the unit “ppm” means “ppm by weight” or “ppmby mass”. Furthermore, in the present specification, “mass” and“weight”, “% by mass” and “% by weight”, and “parts by mass” and “partsby weight” are synonyms, and in regard to the measurement of physicalproperties and the like, measurement is made at room temperature (20° C.to 25° C.)/a relative humidity of 40% to 50% unless otherwise specified.Furthermore, the term “-acid (salt)” means “-acid and/or a saltthereof”, and “(meth)acryl” means “acryl and/or methacryl”.

[2] METHOD FOR PRODUCING WATER ABSORBENT RESIN (2-1) Polymerization Step

The present step is a step of polymerizing an aqueous solutioncontaining acrylic acid and/or a salt thereof (hereinafter, referred toas “acrylic acid (salt)”) as a main component, and thereby obtaining awater-containing gel-like crosslinked polymer.

(a) Monomer (Excluding Crosslinking Agent)

The water absorbent resin obtained by the present invention is usuallypolymerized in the state of an aqueous solution by using, as a rawmaterial thereof (monomer), an aqueous solution containing acrylic acid(salt) as a main component. The monomer concentration (solid contentconcentration) of the aqueous monomer solution is usually 10% to 90% byweight, and preferably 20% to 80% by weight. Further, polymerization ata high monomer concentration (35% by weight or greater, still morepreferably 40% by weight or greater, and particularly preferably 45% byweight or greater; the upper limit is the saturation concentration,still more preferably 80% by weight or less, and particularly preferably70% by weight or less) may be mentioned as a most preferred example.

Furthermore, when the monomer is polymerized in an aqueous solution,surfactants, polymer compounds such as polyacrylic acid (salt), starch,cellulose and polyvinyl alcohol, various chelating agents, and variousadditives may be optionally added, in an amount of 0% to 30% by weight,and preferably 0.001% to 20% by weight, relative to the amount of themonomer.

Furthermore, it is preferable that the hydrogel obtained bypolymerization of the aqueous solution have at least a portion of theacid groups of the polymer neutralized, from the viewpoint of absorptionperformance. The neutralization process can be carried out beforepolymerization (monomer), during polymerization, or after polymerization(hydrogel) of acrylic acid. However, from the viewpoints of productivityof the water absorbent resin, an enhancement of AAP (absorption againstpressure) or SFC (saline flow conductivity), and the like, it ispreferable to carryout neutralization before the polymerization ofacrylic acid. That is, it is preferable to use neutralized acrylic acid(that is, a partially neutralized salt of acrylic acid) as a monomer.

The neutralization ratio of neutralization described above is notparticularly limited, but the neutralization ratio is preferably 10% to100% by mole, more preferably 30% to 95% by mole, still more preferably50% to 90% by mole, and particularly preferably 60% to 80% by mole,relative to the acid group. If the neutralization ratio is less than 10%by mole, the CRC (absorption capacity without load) in particular may bemarkedly decreased, which is not preferable.

Furthermore, in the case of using acrylic acid (salt) as a maincomponent in the present invention, a hydrophilic or hydrophobicunsaturated monomer other than acrylic acid (salt) (hereinafter, alsoreferred to as “other monomer” in some cases) can also be used. Thereare no particular limitations on such another monomer, but examplesinclude methacrylic acid, (anhydrous) maleic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,(meth)acryloxyalkanesulfonic acid, N-vinyl-2-pyrrolidone,N-vinylacetamide, (meth)acrylamide, N-isopropyl (meth)acrylamide,N,N-dimethyl (meth)acrylamide, 2-hydroxyethyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycol(meth)acrylate, stearyl acrylate, and salts thereof, or the like. Whenthese other monomers are used, the use amount is not particularlylimited as long as the use amount is to the extent that the absorptioncharacteristics of the water absorbent resin thus obtainable are notimpaired, but the use amount is preferably 50% by weight or less, andmore preferably 20% by weight or less, relative to the total weight ofmonomers. Further, when these other monomers are optionally used, thelower limit of the use amount is appropriately determined in accordancewith the type, purpose or effect of the monomer and is not particularlylimited; however, the lower limit of the use amount is about 1% byweight relative to the total weight of monomers.

(b) Salt of Neutralization

There are no particular limitations on the basic substance that is usedfor the neutralization of acrylic acid as a monomer or the polymer(hydrogel) after polymerization, but a monovalent basic substanceincluding an alkali metal hydroxide such as sodium hydroxide, potassiumhydroxide and lithium hydroxide, and a (hydrogen) carbonate such assodium (hydrogen) carbonate and potassium (hydrogen) carbonate, and thelike are preferred, or sodium hydroxide is particularly preferred.Furthermore, the temperature at the time of neutralization(neutralization temperature) is not particularly limited, and thetemperature is preferably 10° C. to 100° C., and more preferably 30° C.to 90° C. Meanwhile, in regard to the neutralization treatmentconditions and the like other than those described above, the conditionsand the like disclosed in WO 2007/028751 and U.S. Pat. No. 6,388,000 arepreferably applied to the present invention.

(c) Crosslinking Agent (Internal Crosslinking Agent)

In the present invention, it is particularly preferable to use acrosslinking agent (hereinafter, also referred to as “internalcrosslinking agent” in some cases) from the viewpoint of absorptionperformance of water absorbent resin thus obtained. Examples of theinternal crosslinking agent that can be used include a compound havingtwo or more polymerizable double bonds per molecule, and apolyfunctional compound having two or more functional groups permolecule that are capable of reacting with carboxyl groups and forming acovalent bond therewith. For example, one or more kinds of apolymerizable crosslinking agent capable of polymerizing with acrylicacid, a reactive crosslinking agent capable of reacting with a carboxylgroup, and a crosslinking agent having the two functions thereof incombination, may be exemplified. Specifically, the polymerizablecrosslinking agent may be, for example, a compound having at least twopolymerizable double bonds in the molecule, such asN,N′-methylenebisacrylamide, (poly)ethylene glycol di(meth)acrylate,(polyoxyethylene)trimethylolpropane tri(meth)acrylate, orpoly(meth)aryloxyalkane. Furthermore, examples of the reactivecrosslinking agent include a polyglycidyl ether such as ethylene glycoldiglycidyl ether; a crosslinking agent capable of covalent bonding, suchas a polyhydric alcohol such as propanediol, glycerin, or sorbitol; anda crosslinking agent capable of ionic bonding, such as a polyvalentmetal compound such as an aluminum salt. Among these, from the viewpointof absorption performance, a polymerizable crosslinking agent capable ofpolymerizing with acrylic acid is preferred, and particularly,acrylate-type, aryl-type or acrylamide-type polymerizable crosslinkingagent is suitably used. These internal crosslinking agents may be usedalone, or two or more kinds may be used in combination. The use amountof the internal crosslinking agent described above is preferably 0.001%to 5% by mole, more preferably 0.005% to 2% by mole, still morepreferably 0.01% to 1% by mole, and particularly preferably 0.03% to0.5% by mole, from the viewpoint of the physical properties, relative tothe monomer described above excluding the crosslinking agent.

(d) Other Trace Components

In the present invention, from the viewpoints of color tone stabilityand residual monomers, the content of protoanemonin and/or furfural inacrylic acid is preferably 0 ppm to 10 ppm, more preferably 0 ppm to 5ppm, and still more preferably 0 ppm to 1 ppm. Furthermore, also for thesame reason, the content of aldehyde components other than furfuraland/or maleic acid in acrylic acid is preferably 0 ppm to 5 ppm, morepreferably 0 ppm to 3 ppm, still more preferably 0 ppm to 1 ppm, andparticularly preferably 0 ppm (at or below detection limit). Meanwhile,examples of the aldehyde components other than furfural includebenzaldehyde, acrolein, acetaldehyde, and the like. Further, for thepurpose of reducing the residual monomers, the content of acrylic aciddimer is preferably 0 ppm to 500 ppm, more preferably 0 ppm to 200 ppm,and still more preferably 0 ppm to 100 ppm.

In the present invention, it is preferable that a methoxyphenol compoundbe included in the unsaturated monomer, and it is more preferable thatp-methoxyphenol be included, from the viewpoint of polymerizationstability. The content of the methoxyphenol compound is preferably 1 ppmto 250 ppm, more preferably 5 ppm to 200 ppm, still more preferably 10ppm to 160 ppm, and particularly preferably 20 ppm to 100 ppm, relativeto the monomer (acrylic acid).

(e) Other Component in Aqueous Monomer Solution

In order to improve various physical properties of the water absorbentresin obtained by the present invention, the following substance can beadded as an optional component to the aqueous monomer solution. That is,a water-soluble resin or a water absorbent resin, such as starch,polyacrylic acid (salt), polyvinyl alcohol, or polyethyleneimine can beadded in an amount of, for example, 0% to 50% by weight, preferably 0%to 20% by weight, more preferably 0% to 10% by weight, and still morepreferably 0% to 3% by weight, relative to the monomer. Here, the lowerlimit of the additive amount of the optional component when the optionalcomponent described above is added is appropriately determined inaccordance with the type, purpose and effect of the optional componentsand is not particularly limited, but the lower limit is preferably about0.001% by weight relative to the monomer. Furthermore, additives such asvarious expanding agents (carbonates, azo compounds, air bubbles, andthe like), surfactants, various chelating agents, hydroxycarboxylicacids, and reducing inorganic salts can be added in an amount of, forexample, 0% to 5% by weight, and preferably 0% to 1% by weight, based onthe monomer. Here, the lower limit of the amount of the additive whenthe additive described above is added is appropriately determined inaccordance with the type, purpose and effect of the additive and is notparticularly limited, but the lower limit is preferably about 0.001% byweight relative to the monomer.

Among these, when it is intended to suppress coloration over time of thewater absorbent resin (enhancement of color tone stability in long-termstorage under high temperature and high humidity) or to enhance urineresistance (prevention of gel deterioration), a chelating agent, ahydroxycarboxylic acid, or a reducing inorganic salt is preferably used,and a chelating agent is particularly preferably used. The use amount inthis case is preferably 10 ppm to 5,000 ppm, more preferably 10 ppm to1,000 ppm, still more preferably 50 ppm to 1,000 ppm, and particularlypreferably 100 ppm to 1,000 ppm, relative to the water absorbent resin.Meanwhile, in regard to the chelating agent, hydroxycarboxylic acid andreducing inorganic salt, the compounds disclosed in WO 2009/005114 A, EP2 057 228 B, and EP 1 848 758 B are used.

(f) Polymerization Initiator

The polymerization initiator used in the present invention isappropriately selected depending on the polymerization form, and is notparticularly limited. Examples include a thermally degradable typepolymerization initiator, a photodegradable type polymerizationinitiator, a redox-type polymerization initiator, and the like. Specificexamples of the thermally degradable type polymerization initiatorinclude persulfates such as sodium persulfate, potassium persulfate, andammonium persulfate; peroxides such as hydrogen peroxide, t-butylperoxide, and methyl ethyl ketone peroxide; and azo compounds such as2,2′-azobis(2-amidinopropane)dihydrochloride, and2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and the like.Furthermore, examples of the photodegradable type polymerizationinitiator include benzoin derivatives, benzyl derivatives, acetophenonederivatives, benzophenone derivatives, azo compounds, and the like.Furthermore, examples of the redox-type polymerization initiator includesystems obtained by combining the persulfates or peroxides describedabove with reducing compounds such as L-ascorbic acid and sodiumhydrogen sulfite. According to a preferred embodiment, the thermallydegradable type polymerization initiator and the photodegradable typepolymerization initiator described above may be used in combination. Theuse amount of these polymerization initiators is preferably 0.0001% to1% by mole, and more preferably 0.001% to 0.5% by mole, relative to themonomer described above. If the use amount of the polymerizationinitiator is larger than 1% by mole, it is not preferable becausecoloration of the water absorbent resin may occur. Furthermore, if theuse amount of the polymerization initiator is less than 0.0001% by mole,it is not preferable because there is a risk that the amount of residualmonomers may increase.

Meanwhile, instead of using the polymerization initiator describedabove, polymerization may also be carried out by irradiating an activeenergy ray such as a radiation, an electron beam, or ultravioletradiation. Alternatively, polymerization may also be carried out byusing this active energy ray and the polymerization initiator incombination.

(g) Polymerization Method (Crosslink-Polymerization Step)

In the present invention, at the time of polymerizing the aqueousmonomer solution described above, aqueous solution polymerization orreverse phase suspension polymerization is usually employed from theviewpoints of the absorption performance of the water absorbent resinobtained, the ease of polymerization control, or the like, butpreferably aqueous solution polymerization, and more preferablycontinuous aqueous solution polymerization is employed. Among these,this method is preferably applied to the production in a huge scale witha large production output per one line of the water absorbent resin. Theproduction output is 0.5 [t/hr] or greater, more preferably 1 [t/hr] orgreater, still more preferably 5 [t/hr] or greater, and particularlypreferably 10 [t/hr] or greater.

Furthermore, preferred forms of the aqueous solution polymerizationinclude continuous belt polymerization (U.S. Pat. No. 4,893,999 A, U.S.Pat. No. 6,241,928 A, US 2005/215734 A, and the like), and continuouskneader polymerization (U.S. Pat. No. 6,987,151 A, U.S. Pat. No. 670,141A, and the like).

In regard to the continuous aqueous solution polymerization describedabove, high temperature initiated polymerization in which thepolymerization initiation temperature is set to 30° C. or higher,preferably 35° C. or higher, more preferably 40° C. or higher, stillmore preferably 50° C. or higher, and particularly preferably 60° C. orhigher (the upper limit is the boiling point); or the high monomerconcentration polymerization described above, may be mentioned as one ofmost preferred examples. Meanwhile, the polymerization initiationtemperature described above is defined as the temperature of the liquidimmediately before the supply of the aqueous monomer solution to apolymerization reactor, but the conditions and the like disclosed inU.S. Pat. No. 6,906,159 A, U.S. Pat. No. 7,091,253 and the like can bepreferably applied to the present invention.

Furthermore, from the viewpoints of an enhancement of the physicalproperty of the water absorbent resin obtained and of drying efficiency,it is preferable to obtain a water absorbent resin at a higher solidcontent concentration by evaporating moisture at the time ofpolymerization. The extent of increase in the solid contentconcentration from the aqueous monomer solution (solid content ofhydrogel after polymerization−solid content of monomer beforepolymerization) is preferably 1% by weight or greater, more preferably2% to 40% by weight, and still more preferably 3% to 30% by weight.However, a range in which a water-containing gel-like crosslinkedpolymer having a solid content of 80% by weight or less as will bedisclosed below is obtained is preferred.

These polymerization can be carried out in an air atmosphere, but fromthe viewpoint of preventing coloration, the polymerization is preferablycarried out in an inert gas atmosphere such as nitrogen or argon (forexample, oxygen concentration: 1% by volume or less). Furthermore, it ispreferable to purge the dissolved oxygen in the monomer or the solutioncontaining the monomer with an inert gas (for example, dissolved oxygenconcentration: less than 1 mg/L), and then to perform polymerization.Furthermore, the polymerization can be carried out under any pressure,such as under reduced pressure, under normal pressure, or underpressure.

(2-2) Grain Refining Step for Water-Containing Gel-Like CrosslinkedPolymer (Gel-Crushing Step)

The present step is a step of crushing the water-containing gel-likecrosslinked polymer obtained in the polymerization step described above,and thereby obtaining a particulate water-containing gel-likecrosslinked polymer (hereinafter, referred to as “particulatehydrogel”).

The hydrogel obtained in the polymerization step described above may bedirectly subjected to drying, but preferably, the hydrogel is optionallysubjected to gel-crushing, during polymerization or afterpolymerization, by using a crusher (a kneader, a meat chopper, a cuttermill, or the like) and is converted to a particulate form, in order tosolve the problem described above. That is, a hydrogel grain refining(hereinafter, also referred to as “gel-crushing”) step may further beincluded between the polymerization step based on continuous beltpolymerization or continuous kneader polymerization, and the dryingstep. Meanwhile, even the case in which the gel is subjected to grainrefining by dispersion in a solvent at the time of polymerization, suchas reverse phase suspension polymerization, is also meant to be includedin the grain refining of the present invention (grain refining of thepolymerization step), but suitably, the gel is gel-crushed by using acrusher.

In regard to the temperature of the hydrogel at the time ofgel-crushing, the hydrogel is kept warm or heated preferably to 40° C.to 95° C., and more preferably 50° C. to 80° C., in view of the physicalproperties. The weight average particle size (D50) of the particulatehydrogel after gel-crushing is preferably 0.5 mm to 4 mm, morepreferably 0.5 mm to 3 mm, and still more preferably 0.3 mm to 2 mm.When the weight average particle size (D50) of the particulate hydrogeldescribed above is in the range described above, it is preferablebecause drying can be efficiently carried out. Furthermore, theproportion of the particulate hydrogel having a particle size of 5 mm orlarger is preferably 0% to 10% by weight, and more preferably 0% to 5%by weight, relative to the total amount of the particulate hydrogel.Here, the particle size of the particulate hydrogel is measured by usingthe wet classification method described in paragraph [0091] of JP-A No.2000-63527.

(2-3) Drying Step

The water-containing gel-like crosslinked polymer obtained in thepolymerization step described above, or the particulate hydrogelobtained in the gel-crushing step can be dried up to a desired resinsolid content. There are no particular limitations on the method, butvarious drying methods such as, for example, heating and drying, hot airdrying, drying under reduced pressure, infrared drying, microwavedrying, drum dryer drying, dehydration drying by azeotropy with ahydrophobic organic solvent, and high humidity drying by using watervapor at a high temperature, can be employed. Among these dryingmethods, hot air drying is preferred; hot air drying by a gas having adew point temperature of 0° C. to 100° C. is more preferred; hot airdrying by a gas having a dew point temperature of 20° C. to 90° C. isstill more preferred; and particularly, ventilated band drying ispreferred. Furthermore, the drying temperature (equivalent to the hotair temperature) is also not particularly limited, and the dryingtemperature is preferably 100° C. to 300° C., and more preferably 150°C. to 250° C. However, in order to achieve a balance between animprovement of the physical property of the water absorbent resinobtained and the degree of whiteness, it is preferable that the dryingtemperature be 165° C. to 230° C., and the drying time be within 50minutes, more preferably 20 minutes to 40 minutes. If the dryingtemperature and drying time are not in the ranges described above, thereis a risk of causing a decrease in the absorption capacity without load(CRC) of the water absorbent resin, an increase in the extractables, ora decrease in the degree of whiteness, which is not preferred.

Furthermore, the resin solid content that is determined from the dryweight reduction of the particulate hydrogel (change in the weight when1 g of a water absorbent resin powder or particle is heated for 3 hoursat 180° C.) is preferably 80% by weight or greater, more preferably 85%to 99% by weight, still more preferably 90% to 98% by weight, andparticularly preferably 92% to 97% by weight. In the present dryingstep, a dried product in which the resin solid content has been adjustedto the range described above is obtained. The resin solid content of thedried product substantially corresponds to the resin solid content ofthe water absorbent resin powder before being surface crosslinked, andis preferably applied as the resin solid content for the classificationstep in the present invention.

When the moisture content of the dried product obtained in the presentstep is high, that is, when the resin solid content is low, for example,when the resin solid content is less than 80% by weight, a decrease inthe absorption capacity (per the resin solid content), aggregation ofthe water absorbent resin in a subsequent step such as the surfacecrosslinking step, a decrease in the physical property, and further adecrease in conveyance property may be observed. On the other hand, whenthe moisture content of the dried product obtained in the present stepis low, that is, the resin solid content is high, for example, when theresin solid content is greater than 99% by weight, the water absorbentresin may not be obtained unless a long time is taken as the dryingtime, and also, deterioration of the water absorbent resin or a decreasein the powder characteristics (prevention of static charge, impactresistant stability, deterioration of the physical property at the timeof transport, and the like) may be observed.

Moisture (moisture content) in the water absorbent resin becomes aninhibitory factor of classification, but it is preferable to produce awater absorbent resin having a moisture content in the predeterminedrange described above, rather than to produce a water absorbent resin inan absolute dry state with a moisture content of less than 1% by weight.Since such a water absorbent resin can exhibit a better effect through aclassification step, it can be preferably applied to a method forproducing a water absorbent resin, which includes a classification stepof a water absorbent resin having a moisture content in the rangedescribed above.

Furthermore, in order to promote a reduction of residual monomers of thewater absorbent resin obtained, prevention of gel deterioration (urineresistance), and prevention of yellowing, it is preferable to shortenthe time taken from after the completion of polymerization to theinitiation of drying. That is, regardless of the presence or absence ofthe gel-crushing step described above, it is preferable to adjust thetime period from the point of completion of polymerization to theinitiation of drying, to one hour or less, more preferably 0.5 hours orless, and still more preferably 0.1 hours or less. Furthermore, duringthis period, the temperature of the water-containing gel-likecrosslinked polymer is preferably controlled to 50° C. to 80° C., andstill more preferably 60° C. to 70° C. When the temperature iscontrolled to this temperature range, a reduction of residual monomersor low coloration can be achieved.

(2-4) Pulverization Step

The present step is a step for pulverizing the dried product obtained inthe drying step described above, and is an optional step in the presentinvention. The pulverizer that can be used in the present step is notparticularly limited, and any conventionally known pulverizer can beused. Specific examples include a roll mill, a hammer mill, a rollgranulator, a jaw crusher, a gyratory crusher, a cone crusher, a rollcrusher, a cutter mill, and the like. Among these, from the viewpoint ofparticle size control, it is preferable to use a multistage roll mill ora roll granulator. Meanwhile, in the case of drying the particulatewater-containing gel-like polymer (particularly drying by using aventilated band type dryer), the particulate hydrogel aggregates duringthe drying period, and a block-like dried product (aggregate) may beformed. In this case, it is desirable to perform coarsely crushing ofthe aggregate (an operation of breaking aggregation).

Through the pulverization step, the dried product obtained in the dryingstep described above is pulverized, and thus a pulverized dried product(an irregularly shaped crushed water absorbent resin powder) isobtained. Since the physical property of the water absorbent resinpowder is improved by the pulverization step, the pulverization step ispreferably applied.

From the viewpoint of enhancing the physical property of the waterabsorbent resin obtained in the present step, it is preferable tocontrol the particle size to obtain the particle size described below.That is, the weight average particle size (D50) of the water absorbentresin powder (before surface crosslinking) is preferably 200 μm to 600μm, more preferably 200 μm to 550 μm, still more preferably 250 μm to500 μm, and particularly preferably 350 μm to 450 μm. Furthermore, theproportion of fine particles that pass through a sieve (JIS standardsieve) having a mesh opening size of 150 μm (having a particle size ofless than 150 μm) is preferably 0% to 5% by weight, more preferably 0%to 3% by weight, and still more preferably 0% to 1% by weight, relativeto the total amount of the water absorbent resin powder. Furthermore,the proportion of giant particles that do not pass through a sieve (JISstandard sieve) having a mesh opening size of 850 μm (having a particlesize of 850 μm or greater) is preferably 0% to 5% by weight, morepreferably 0% to 3% by weight, and still more preferably 0% to 1% byweight, relative to the total amount of the water absorbent resinpowder. Further, the logarithmic standard deviation (σζ) of the particlesize distribution is preferably 0.20 to 0.40, more preferably 0.25 to0.39, and still more preferably 0.27 to 0.38. These physical propertyvalues are measured by using standard sieves and by the methodsdisclosed in, for example, WO 2004/069915 A or EDANA-ERT420.2-02(“PSD”). Furthermore, in the present invention, the proportion ofparticles having a particle size of equal to or greater than 150 μm andless than 850 μm is preferably 95% by weight or greater, and morepreferably 98% by weight or greater, relative to the total amount of thewater absorbent resin powder, and the upper limit is 100% by weight. Itis preferable to surface crosslink a water absorbent resin powder havingthe proportion described above. Furthermore, also for the particle sizeof the water absorbent resin after being surface crosslinked, andfurther the particle size of the final product, the same particle sizeas that of the water absorbent resin powder before being surfacecrosslinked described above is applied.

(2-5) Classification Step

The present step is a step of adjusting the particle size to aparticular particle size (weight average particle size (D50), particlesize distribution, or the like), in order to improve the physicalproperty of the water absorbent resin. Meanwhile, the particle sizecontrol is not limited to the present classification step, and can beappropriately adjusted in a polymerization step (particularly, reversephase suspension polymerization), a pulverization step, a fine powdercollection step, a granulation step, and the like. Hereinafter, theparticle size will be defined by standard sieves (JIS Z8801-1 (2000) orequivalents thereof).

In the present step, when sieve classification is carried out, and atleast one of the following constitutions (i) and (ii) is carried out,the physical property (particularly, liquid permeability) of the surfacecrosslinked water absorbent resin is improved. Furthermore, even incontinuous production for a long time period, a decrease in the physicalproperty (particularly, liquid permeability) can be reduced. Meanwhile,evaluation of physical property values in continuous production iscarried out in the Examples described below, and generally in anoperation for a long time, it is observed that the physical propertytends to deteriorate with a lapse of time. However, such a tendency isnot seen in the present invention, and such an effect is markedlyexhibited at the time of industrial continuous production of a waterabsorbent resin having high liquid permeability, particularly at thetime of industrial continuous production (for example, continuousproduction for 24 or more hours at 1 [t/hr]) of a water absorbent resinhaving an SFC (saline flow conductivity) of 10[×10⁻⁷ cm³·s·g⁻¹] or more.

The water absorbent resin powder obtained in the pulverization stepdescribed above is classified before and/or after the surfacecrosslinking step that will be described below, and preferablyclassified before and after the surface crosslinking step, andparticularly preferably, the water absorbent resin powder is sieveclassified. Through the classification step, a classified product (waterabsorbent resin powder) having a desired particle size described aboveis obtained. When the classification step is carried out before thesurface crosslinking step, the final product can be adjusted to have aparticle size in the desired range, which is preferable. Furthermore,when the classification step is carried out after the surfacecrosslinking step, aggregate particles having a particle size other thanthe desired size, which are generated at the time of mixing of a surfacecrosslinking agent or at the time of heating treatment, or fineparticles having a particle size other than the desired size, which aregenerated by physical and mechanical destruction in these steps, areremoved by classification and further disintegrated, and thereby a waterabsorbent resin having excellent performance can be produced, which ispreferable.

(a) Conventional Classification Method

Methods for sieve classification of a water absorbent resin aredisclosed in, for example, Patent Literature 24 to 29 described below.These Patent Literature 24 to 29 suggest neither the constitutions (i)to (v) described below, nor the problems or effects of the presentinvention. Furthermore, Patent Literature 1 to 23 described above do notdisclose the classification method of the present invention.

Furthermore, Patent Literature 30 (International filing date: Sep. 11,2009), which was internationally filed before the priority date of thepresent application, discloses that removal of electricity is carriedout in a classification step for a water absorbent resin, but does notdisclose or suggest the constitutions (i) to (v) of the presentinvention as described below.

-   Patent Literature 24: U.S. Pat. No. 6,164,455 A-   Patent Literature 25: WO 2006/074816 A-   Patent Literature 26: WO 2008/037672 A-   Patent Literature 27: WO 2008/037673 A-   Patent Literature 28: WO 2008/037675 A-   Patent Literature 29: WO 2008/123477 A-   Patent Literature 30: WO 2010/032694 A

(b) Production Method, Classification Method, and Liquid PermeabilityEnhancing Method of the Present Invention

The method for producing a water absorbent resin according to thepresent invention is a method for producing a water absorbent resin,which includes a polymerization step of polymerizing an aqueous solutionof acrylic acid (salt) to obtain a water-containing gel-like crosslinkedpolymer; a drying step of drying the water-containing gel-likecrosslinked polymer to obtain a water absorbent resin powder; aclassification step of classifying the water absorbent resin powder; anda surface crosslinking step of surface crosslinking the water absorbentresin powder, and the method needs to satisfy any one or more of thefollowing constitutions (i) and (ii). Furthermore, it is preferable thatthe method satisfy the following constitutions (iii) to (v), in additionto the following constitutions (i) and/or (ii).

(i) The stretch tension (tension) of the metal sieve mesh used in theclassification step is from 35 [N/cm] to 100 [N/cm].

(ii) An airbrush is used below the metal sieve mesh used in theclassification step.

(iii) A tapping material is used below the metal sieve mesh used in theclassification step.

(iv) A classification aid particle is used, and the classification aidparticle and a fine powder (fine particle) of the water absorbent resinare removed.

(v) Damage monitoring of a metal sieve mesh that is used in theclassification step is carried out (preferably by electrical detectionusing ultrasonic waves or high frequency waves, particularly by an AEmethod).

Furthermore, it is preferable that the metal sieve be further heated orkept warm. It is preferable that the metal sieve be removed ofelectricity. Classification is preferably carried out under reducedpressure conditions.

Hereinafter, the constitutions (i) to (v) will be described in detail.

(i) Stretch Tension (Tension)

The term “stretch tension (tension)” according to the present inventionmeans the load imposed when the metal sieve mesh used in theclassification step is stretched. The tension of the classifying mesh(metal mesh) of the present invention is preferably 35 [N/cm] orgreater, more preferably 40 [N/cm] or greater, still more preferably 45[N/cm] or greater, and particularly preferably 50 [N/cm] or greater.Furthermore, the upper limit of the tension is preferably 100 [N/cm] orless, more preferably 80 [N/cm] or less, and still more preferably 60[N/cm] or less. When the tension describe above is 35 [N/cm] or greater,a decrease in the classification efficiency of the water absorbent resinpowder can be prevented, and liquid permeability of the water absorbentresin obtained is improved. Furthermore, when the tension describedabove is 100 [N/cm] or less, since durability of the metal mesh can besecured, continuous operation is enabled. Meanwhile, the measurement ofthe tension described above is carried out by using a tension meter atthe center of the sieve when the metal mesh is stretched in theclassifying sieve. The principle of measurement is “Mechanical measuringof the fabric's sagging under a constant force.” The tension meterdescribed above is such that various products are commerciallyavailable, and for example, products are sold from Tekomat SE and thelike. In the present invention, those commercially available productscan be used.

In the method according to the present invention for producing a waterabsorbent resin which includes a classification step, it was found thatthe stretch tension (tension) of the metal sieve mesh affects thephysical property of the water absorbent resin, particularly thephysical property of the water absorbent resin after being surfacecrosslinked, in particular the absorption capacity under load (forexample, AAP) or liquid permeability (for example, SFC). It was foundthat such stretch tension (tension) also affects the case where theorganic surface crosslinking agent and the inorganic surfacecrosslinking agent are used in the (2-6) surface crosslinking stepdescribed below, and further, an irregularly shaped crushed waterabsorbent resin including the (2-4) pulverization step described above.Thus, the inventors completed the present invention.

The technologies for enhancing the absorption capacity under load orliquid permeability have been suggested in many literature such asPatent Literature 1 to 23 described above; however, in these related artliterature, there is no disclosure of a technology which paid attentionto the classification step, and in particular, to the stretch tension(tension) of the metal sieve mesh. Meanwhile, since not only anenhancement of the above-mentioned physical property (particularly, theabsorption capacity under load or liquid permeability), but also anincrease in the productivity of a water absorbent resin having theparticular physical property described above or prevention of operationtrouble (stopped production due to the destruction of the sieve mesh)can be promoted by the control of the stretch tension (tension)according to the present invention, it is more preferable. When thestretch tension (tension) is controlled to a certain extent, there isless damage to the sieve mesh during continuous operation, andcontinuous operation for a long time is enabled. Conventionally, therehave been occasions in which when continuous operation for a long timeis implemented, a rapid decrease in the physical property or a change inthe particle size is recognized. However, when the cause wasinvestigated, it was found that damage of the sieve in theclassification step is a cause, and the problem could be solved bycontrolling tension in the classification step.

Furthermore, in the case of using one or plural classifying meshes(metal sieve mesh) in a single classification step (classifyingapparatus), and/or in the case of performing classification in two ormore plural classification steps (for example, classification step 1(before the surface crosslinking step), classification step 2 (after thesurface crosslinking step), and classification step 3 (after the surfacecrosslinking step) before and after the surface crosslinking step) byusing one or plural classifying meshes (metal sieve mesh), the controlof the stretch tension (tension) of the metal sieve mesh need beimplemented in some of the metal sieve mesh, or in all of the metalsieve mesh. That is, in the classification step that are carried outbefore the surface crosslinking step and/or after the surfacecrosslinking step, it is preferable to control the stretch tension(tension) in 30% or more of the total number of the metal sieve meshused in the classification step, and preferably, in sequence, in 50% ormore, 70% or more, 80% or more, and 90% or more of the metal sieve mesh.It is most preferable to control the stretch tension (tension) insubstantially 100% of the metal sieve mesh.

The method of stretching the metal mesh is not particularly limited, buta metal mesh may be placed over a sieve frame, and while one edge of themetal mesh is pulled with a jack or the like to be planar, the metalmesh may be fixed to the sieve frame by soldering or with bolts. Thetension according to the present invention is defined as the tension atthe time of fabrication of a metal sieve. For the metal mesh, a meshmade of a metal as will be described below, preferably a mesh made ofstainless steel (preferably, SUS304, SUS316) or made of Magnestain, isused.

The tension of the classifying mesh described above may also be used incombination with the following constitution (ii) or (iii). Further, thetension may be used in further combination with the constitutions (iv)and/or (v).

(ii) Airbrush (Air Knife)

From the viewpoints of the classification efficiency for the waterabsorbent resin powder and the physical property of the water absorbentresin obtained, an airbrush (air knife) is used below the metal sievemesh described above. According to the present invention, the airbrushrefers to an instrument which sprays gas (air) such as compressed air,and is also called an air knife.

Examples of the airbrush (air knife) described above include an air jetcleaner, an air jet brush cleaner, and the like.

In regard to the method according to the present invention for producinga water absorbent resin including a classification step, it was foundthat an airbrush (air knife) affects the physical property of the waterabsorbent resin, particularly the physical property of the waterabsorbent resin after being surface crosslinked, in particular, theabsorption capacity under load (for example, AAP) or liquid permeability(for example, SFC). It was found that such an airbrush (air knife)affects the case where the organic surface crosslinking agent and theinorganic surface crosslinking agent are used in the (2-6) surfacecrosslinking step described below, and further, an irregularly shapedcrushed water absorbent resin including the (2-4) pulverization stepdescribed above. Thus, the inventors completed the present invention.

Technologies for enhancing the absorption capacity under load or liquidpermeability have been suggested in many Literature such as PatentLiterature 1 to 23 described above; however, in these related artLiterature, there is no disclosure of a technology which paid attentionto the classification step, and in particular, an airbrush (air knife).Meanwhile, since not only an enhancement of the above-mentioned physicalproperty (particularly, the absorption capacity under load or liquidpermeability), but also an increase in the productivity of a waterabsorbent resin having the particular physical property described abovecan be promoted by the control of the airbrush (air knife) according tothe present invention, it is still more preferable.

Furthermore, in the case of using plural classifying meshes (metal sievemesh) in a single classification step (classifying apparatus), and/or inthe case of performing classification in two or more pluralclassification steps (for example, classification step 1 (before thesurface crosslinking step) or classification step 2 (after the surfacecrosslinking step) before and after the surface crosslinking step), theuse of an airbrush (air knife) may be implemented in some of the sieves,or may be implemented in all of the sieves. However, the airbrush ispreferably used with at least some of the sieves (sieves having a meshopening size of 300 μm or less), and it is preferable to use theairbrush (air knife) with 30% or more of all the sieves, and preferably,in sequence, in 50% or more, 70% or more, 90% or more, and 100% of allthe sieves.

When the tapping material is used above the metal sieve mesh, or thetapping material is not used, there occurs a problem of deterioration,particularly deterioration over time, of the physical property of thewater absorbent resin (for example, liquid permeability), or an increasein fine powder or powder dust, which is not preferable. However, in thiscase, air that is sprayed from a rotating nozzle arm may be sprayedstrongly from the back surface (particularly, the back surface of acircular sieve) of the metal sieve mesh, to perform cleaning.

The airbrush of the present invention is particularly preferably used atthe time of classifying fine powder, and it is preferable that theairbrush be installed below a sieve having a mesh opening size of 200 μmor less, and more preferably a sieve having a mesh opening size of 150μm or less. The lower limit of the mesh opening size of the sievedescribed above is preferably 30 μm or greater, more preferably 45 μm orgreater, and still more preferably 75 μn or greater. The airbrush ispreferably used for the classification of fine powder as a substitutefor the tapping material describe above, and can enhance the liquidpermeability (SFC or GBP) of the water absorbent resin obtained.

From the viewpoint that the water absorbent resin powder can stablyretain excellent physical property, and the blocking phenomenon can besuppressed, it is preferable that the airbrush of the present inventionuse dry air as primary air and secondary air. The dew point of the dryair is preferably 0° C. or lower, more preferably −30° C. or lower,still more preferably −35° C. or lower, and particularly preferably −40°C. or lower. Examples of methods of controlling the dew point include amethod of using a membrane dryer, a method of using a cooling adsorptiondryer, a method of using a diaphragm dryer, and methods of using thesedryers in combination. In the case of using an adsorption dryer, theadsorption dryer may be a heating regeneration type, a non-heatingregeneration type, or a non-regeneration type.

Furthermore, heated air may also be used, other than dry air. Theheating method for the heated air is not particularly limited, but airmay be directly heated by using a heat source, or may be indirectlyheated by heating an apparatus or pipes. The temperature of the heatedair is preferably 30° C. or higher, more preferably 50° C. or higher,and still more preferably 70° C. or higher.

The airbrush described above is preferably used in combination with thetension of the constitution (i) described above. Further, the airbrushis preferably used in further combination with the constitution (iv)and/or (v).

(iii) Tapping Material

From the viewpoints of the classification efficiency of the waterabsorbent resin powder and the physical property of the water absorbentresin obtained, a tapping material is used below the metal sieve meshdescribed above. A tapping material refers to an elastic material thatis used to prevent clogging of a sieving apparatus, and for the shape ofthe tapping material, any shape that is capable of rolling, such as asphere, a spheroid, or a polyhedral, can be utilized. Preferably, atleast one selected from a tapping ball (spherical), a tapping block(spherical) and a tapping brush is used, and more preferably, a tappingball or a tapping block, still more preferably a tapping ball, is used.Meanwhile, when the tapping material is used above the metal sieve mesh,or the tapping material is not used, there may occur a problem ofdeterioration, particularly deterioration over time, of the physicalproperty of the water absorbent resin (for example, liquidpermeability), or an increase in fine powder or powder dust, which isnot preferable.

In the present invention, it was found that in the method for producinga water absorbent resin including a classification step, a tappingmaterial (preferably, a heated tapping material) affects the physicalproperty of the water absorbent resin, particularly the physicalproperty of the water absorbent resin after being surface crosslinked,in particular, the absorption capacity under load (for example, AAP) orliquid permeability (for example, SFC). It was found that such a tappingmaterial affects the case where the organic surface crosslinking agentand the inorganic surface crosslinking agent are used in the (2-6)surface crosslinking step described below, and further, an irregularlyshaped crushed water absorbent resin including the (2-4) pulverizationstep described above. Thus, the inventors completed the presentinvention.

The technologies of enhancing the absorption capacity under load orliquid permeability have been suggested in many literature such asPatent Literature 1 to 23 described above; however, in these related artliterature, there is no disclosure of a technology which paid attentionto the classification step, and in particular, the tapping material.Furthermore, not only the physical property described above(particularly, the absorption capacity under load or liquidpermeability) are enhanced, but also productivity of a water absorbentresin having the particular physical properties described above can beenhanced by the control of the tapping material of the presentinvention, and therefore, it is still more preferable.

Further, in the case of using plural classifying meshes (metal sievemeshes) in a single classification step (classifying apparatus), and/orin the case of performing classification in two or more pluralclassification steps (for example, classification step 1 (before thesurface crosslinking step) and classification step 2 (after the surfacecrosslinking step) before and after the surface crosslinking step), thetapping material may be used with some of the sieves, or may be usedwith all of the sieves. However, it is preferable to use the tappingmaterial with at least some of the sieves (sieves having a mesh openingsize of 300 μm or less), and preferable to use the tapping material with30% or more of all the sieves, and in sequence, with 50% or more, 70% ormore, 90% or more, and 100% of all the sieves.

The method of using the tapping material of the present invention belowa metal sieve mesh is not particularly limited, but for example, amethod of further providing, below the metal sieve mesh, a metal sievemesh having a mesh opening size equal to or larger than the mesh openingsize of the metal sieve mesh, or a punching metal having a hole diameterequal to or larger than the mesh opening size of the metal sieve mesh,and packing a tapping material (preferably, a tapping ball or a tappingblock) on this metal sieve mesh or punching metal, may be used. From theviewpoint of classification efficiency, it is preferable to use thetapping material on a punching metal.

The tapping material described above is preferably made of a resin, andexamples thereof include natural rubber, urethane, chloroprene rubber, asilicone resin, and the like. Among these, it is preferable to use awhite or milky white tapping material, particularly natural whiterubber, white urethane, or the like, in consideration of adherence,incorporation or the like of the tapping material to white waterabsorbent resins. Meanwhile, the compressive elastic modulus (Young'smodulus) of these resins is preferably 0.05 GPa to 1.0 GPa, and morepreferably 0.06 GPa to 0.5 GPa.

Further, the size and shape of the tapping material may be appropriatelydetermined in accordance with desired physical properties of the waterabsorbent resin, but the shape is preferably a block shape or aspherical shape, while the size (diameter) is preferably 5 mm to 200 mm,still more preferably 10 mm to 100 mm, and particularly preferably 20 mmto 60 mm. Furthermore, when the size is in the ranges described above,tapping balls or taping blocks having different sizes may be used incombination. Meanwhile, when a tapping block is used, the volume iscalculated in terms of the volume of a sphere, and thus the size isdetermined.

In the present invention, it is preferable to use plural tappingmaterials (tapping balls, tapping blocks or the like). The use amount ofthe tapping material of the present invention is defined as thecross-sectional area of tapping balls relative to the area of the metalsieve mesh, and the amount is preferably 1% or greater, and preferably,in sequence, 5% or greater, 10% or greater, 15% or greater, and 20% orgreater. The upper limit is preferably less than the closest packing inconsideration of the gaps between the tapping balls, and more preferably70% or less. The use amount may be appropriately determined in thisrange.

The water absorbent resin powder that has been classified by using atapping material below the metal sieve mesh passes through the metalsieve mesh or punching metal, preferably the punching metal, packed(loaded) with the tapping material, and may be supplied to thesubsequent step (see FIG. 2).

In the sieve classifying apparatus illustrated in FIG. 2, a punchingmetal 24 is disposed below each of three metal sieve meshes 21 to 23having different mesh opening sizes, and tapping balls 25 are packed onthis punching metal 24. The mesh opening sizes of the three metal sievemeshes 21 to 23 decrease stage by stage from top to bottom, and forinstance, the mesh opening sizes of the three metal sieve meshes 21, 22and 23 are 1,000 μm, 850 μm, and 100 μm, respectively.

Here, since the metal sieve mesh or punching metal packed (loaded) withthe tapping material is installed below the metal sieve mesh used forthe classification of the water absorbent resin, it is preferable thatthe shape be substantially the same as that of the metal sieve mesh. Forexample, when the metal sieve used for the classification of the waterabsorbent resin is circular in shape, it is preferable that the metalsieve mesh or punching metal packed (loaded) with the tapping materialbe similarly circular in shape.

The hole diameter of the punching metal described above is smaller thanthe tapping material, and is desirably 5 mm or greater, and morepreferably 10 mm or greater. Meanwhile, there are no particularlimitations on the upper limit of the hole diameter of the punchingmetal, but the hole diameter is preferably 30 mm or less. Furthermore,from the viewpoint of classification efficiency, it is preferable thatthe punching metal have a hole diameter that is 5 times or more the meshopening size of a metal sieve mesh. More preferably, the punching metalhas a hole diameter that is from 6 times to 20 times the mesh openingsize of a metal sieve mesh.

The hole ratio of the punching metal is preferably 15% to 50%, morepreferably 20% to 45%, and still more preferably 25% to 40%. Meanwhile,the hole ratio is defined by the hole, pitch (P) and the like, but inthe case where there are no holes in certain regions, for example, inthe case where the punching metal has a rim, the hole ratio of thepunching metal is defined as an area which also includes that regions.When the hole ratio is not in the range described above, there is atendency that the physical property of the water absorbent resindeteriorate and the classification efficiency also decreases.

Furthermore, the distance (interval) between the sieve disposed aboveand the metal mesh (punching metal) installed below may be appropriatelydetermined; however, in view of the effects of the present invention,the distance is usually preferably 1.1 to 5 times, more preferably 1.2to 3 times, and still more preferably 1.3 to 2 times, relative to thediameter of the tapping material. If the interval described above is notin this range, there is a tendency that the physical property of thewater absorbent resin obtained deteriorate and the classificationefficiency decreases, which is not preferable.

Furthermore, in the present invention, the tapping material is providedon a punching metal that is disposed below a metal sieve mesh, but it ispreferable that the tapping material be provided over the entire area ofa punching metal in the plane direction, or on a punching metal that isdivided into plural compartments (see FIG. 3).

As illustrated in FIG. 3, when a punching metal divided into pluralcompartments (segmented screens) is used, the method of dividing thesections may be appropriately determined and is not particularlylimited. However, for example, in the case of a circular punching metal,the punching metal may be segmented into two sections, four sections oreight sections, or the center area may be further divided into acircular region. The punching metal may also be segmented into 2 to 100sections, preferably 4 to 50 sections, or more preferably 8 to 40sections, by combining the ways to divide the punching metal. Meanwhile,the size or shape of each room, and the tapping material provided ineach room may all be identical or may be different.

The tapping material (ball) described above is preferably used incombination with the tension of the constitution (i) described above.Further, preferably, the tapping material is used in further combinationwith the constitution s (iv) and/or (v) described below.

In the present invention, it is preferable that the tapping material beheated at the time of classification, from the viewpoints of anenhancement of the physical property of the water absorbent resinobtained and an enhancement of productivity. The heating temperature ispreferably 40° C. or higher, more preferably 50° C. or higher, and stillmore preferably 60° C. or higher. The upper limit of the heatingtemperature is appropriately set, but there is a risk that excessiveheating may reduce the effect of the tapping material, and the servicelife of the tapping material may be shortened. Therefore, the heatingtemperature is usually preferably 150° C. or lower, more preferably 120°C. or lower, still more preferably 100° C. or lower, particularlypreferably 90° C. or lower, and most preferably 80° C. or lower.Accordingly, the temperature of the tapping material may be selected tobe, for example, 40° C. to 100° C., 50° C. to 100° C., 60° C. to 100°C., or the like. However, the temperature is not intended to be limitedto this range, and is defined to be in any arbitrary range selected fromthe upper limit value and the lower limit value of the heatingtemperature described above.

In order to heat the tapping material according to the present inventionto the temperature range described above, the tapping material may beheated from an external source. As the heat source, the interior of thesieve, the surface of the sieve, and the water absorbent resin may beheated to a predetermined temperature, and the contact time or theamount of contact with the tapping material (for example, the flow rateof hot air to the sieve, the flow rate or retention rate of the waterabsorbent resin on the sieve, or the like) may be controlled.

In the present invention, since the tapping material wears out as theoperating time elapses, it is preferable to regularly replace thetapping material depending on the wear of the tapping material. The wearof the tapping material can be monitored by means of, for example, thedecrement of the diameter of the sphere, and the tapping material may bereplaced at the time point when the decrement of the diameter reaches 3%or more, preferably 5% or more, more preferably 10% or more, and stillmore preferably 20% or more. If the tapping material is not regularlyreplaced, the physical properties of the water absorbent resin maygradually deteriorate with the lapse of the operating time. Further, thetime (period) for replacement may be appropriately determined, but thetapping material may be replaced preferably after a substantiallycontinuous operation for 30 days to 2 years, and more preferably 60 daysto one year. Meanwhile, the term “substantially continuous operation”means a state in which, even in the case of including some resting orconversion periods, continuous operation is carried out for 80% or more,90% or more, or 95% or more, of the operating period.

(iv) Classification Aid Particle

In the classification step of the present invention, a classificationaid particle other than the water absorbent resin may also be used. Thatis, in the classification step that is carried out before the surfacecrosslinking step and/or after the surface crosslinking step, the waterabsorbent resin powder and a classification aid particle may beintroduced to the metal sieve mesh used in the classification step.Preferably, from the viewpoints of the classification efficiency and thephysical property of the water absorbent resin, the water absorbentresin powder and a classification aid particle having a specific gravitydifferent from that of the water absorbent resin powder are introducedto the metal sieve mesh used in the classification step, and a finepowder of the water absorbent resin and the classification aid particleare removed. There are no particular limitations on the classificationaid particles, but an inorganic fine particle or an organic fineparticle, which has a larger specific gravity as compared with the waterabsorbent resin powder, may be used. Preferably, an inorganic fineparticle, and more preferably, a water-insoluble inorganic fine particleis used. At the time of classifying the water absorbent resin powder,the classification aid particle is mixed therewith, and afterclassification, particularly after sieve classification, fine particlesof the water absorbent resin and the classification aid particle areremoved in a mixed state. The classified water absorbent resin fineparticles and the classification aid fine particles can be optionallyseparated, and then reused.

The classification aid particle used in the present invention is suchthat particle having a specific gravity larger than the specific gravityof the water absorbent resin powder (in the case of sodium polyacrylate,about 1.6 [g/cm³]), and usually, classification aid particle having aspecific gravity of 2 [g/cm³] or greater, more preferably 2.0 to 4.0[g/cm³], still more preferably 2.3 to 3.5 [g/cm³], and particularlypreferably 2.5 to 3.0 [g/cm³], is used. Furthermore, the apparentspecific gravity (volume specific gravity) thereof is preferably 0.5[g/cm³] or greater.

The inorganic fine particle used, particularly water-insoluble fineparticle, is preferably a powder of a water-insoluble polyvalent metalsalt or a hydroxide or oxide thereof, and more preferably a powder of awater-insoluble polyvalent metal salt. Examples include calcium saltsand aluminum salts, and calcium carbonate (2.711 [g/cm³]) (calcite),2.93 [g/cm³]), calcium sulfate (2.96 [g/cm³] (anhydride), 2.32 [g/cm³](dihydrate)), calcium oxide (3.35 [g/cm³]) and the like are used. Forexample, calcium carbonate produced by pulverizing limestone is referredto as heavy calcium carbonate, and can be graded on the basis of thesize of the particles. If necessary, calcium carbonate may be surfacetreated. There are no particular limitations on the organic fineparticle, but examples include fluororesins and the like.

The particle size of the classification aid particle is preferably 100μm or less, more preferably 10 μm or less, and still more preferably 1μm or less. The lower limit is preferably 10 nm or greater, and stillmore preferably 50 nm or greater. The surface of the classification aidparticle may be coated with a substance such as a fatty acid, rosinacid, lignin acid, or a quaternary ammonium salt, and the use amount ofthese substances is preferably 0.01% to 100% by weight, more preferably0.1% to 10% by weight, and still more preferably 0.5% to 5% by weight,relative to the classification aid particles. Meanwhile, for the purposeof enhancing the anti-caking property, liquid permeability anddiffusibility, methods of mixing a water absorbent resin with inorganicfine particles of silica, kaolin or the like (WO 92/18171 A, EP 629411A, U.S. Pat. No. 6,124,391, JP 59-80459 A, and the like) are known.However, according to the present invention, the classification aidparticle (preferably, 10% by weight or more of the classification aidparticles added, and preferably, in sequence, 30% by weight or more, 50%by weight or more, 70% by weight or more, and particularly preferably90% by weight or more) is removed together with fine particles of thewater absorbent resin, for the purpose of enhancing the classificationefficiency of the water absorbent resin powder or enhancing liquidpermeability of the water absorbent resin obtained. Meanwhile, theproportion of the classification aid in the mixture of theclassification aid particle and the water absorbent resin fine particleis measured by appropriately employing a measurement method depending onthe classification aid particles used.

The classification aid particle of the present invention is preferablyused in the classification step before and/or after surfacecrosslinking, and more preferably after surface crosslinking. Further,the classification aid particle is preferably used at the time ofclassification of fine powder in particular, as in the case of the (c)airbrush described above, and it is preferable that the classificationaid particle be added over a sieve having a mesh opening size of 200 μmor less, and more preferably a sieve having a mesh opening size of 150μm or less. The lower limit of the mesh opening size of the sievedescribed above is preferably 30 μm or larger, more preferably 45 μm orlarger, and still more preferably 75 μm or larger. The classificationaid particle is preferably used for the classification of fine powdersof a water absorbent resin of a specific range, so as to promote anenhancement of liquid permeability (SFC or GBP).

The classification aid particle described above is preferably used incombination with the (i) tension described above. It also may be used incombination with any one of the (ii) airbrush below the sieve and the(iii) tapping material below the sieve.

(v) Damage Monitoring of Classifying Mesh

In the present invention, it is preferable to perform classificationwhile the presence or absence of damage to the classifying mesh (metalsieve mesh) is steadily monitored. That is, according to the presentinvention, it is preferable to monitor the presence or absence of damageto the metal sieve mesh used in the classification step, in theclassification steps that are carried out before the surfacecrosslinking step and after the surface crosslinking step. By steadilymonitoring damage of the sieve mesh, any deterioration of the physicalproperty can be instantly checked, and substandard products can besuppressed to a minimum level even in huge scale production. There areno particular limitations on the method of checking damage of theclassifying mesh, but examples of the means for detecting destruction ofthe classifying mesh include methods of electrically detectingdestruction by using ultrasonic waves or high frequency waves (forexample, WO 2004/045198 A, JP 11-290781 A, or the like); and methods ofdetecting changes in the electrostatic capacity (for example, JP04-281882 A). Furthermore, as an example of a suitable mesh destructiondetecting system, also available is a method of attaching a Centriscan™(Nishimura Machine Works Co., Ltd.), which automatically senses damage(breakage) of the classifying mesh by using microwaves, to a circularsieve machine that has been heated or kept warm.

A suitable method of using ultrasonic waves may be a non-destructiveinspection method, and specific examples thereof include an AE (acousticemission) method and an ultrasonic flaw detection method. The ultrasonicflaw detection method is a method of analyzing a signal produced byultrasonic waves that have been transmitted and reflected by defects.

Conventionally, damage to the sieve mesh has been detected based on thechanges in the particle size of the water absorbent resin. However, inhuge scale production (particularly, continuous production at a rate of1 [t/hr] or more), a significant change in the particle sizedistribution is detected, and there was even an occurrence in which alarge quantity of substandard products were produced without recognizingdamage to the sieve mesh. Examples of the conventional method describedabove include a detection method disclosed in WO 2008/037674 A, which isbased on a method of irradiating electromagnetic waves to the mass flowrate of a carrier that flows at a rate of at least 0.1 [m/s].

On the contrary to this conventional detection method, in the presentinvention, damage to the sieve mesh can be instantly determined bydirectly inspecting the damage to the sieve mesh by ultrasonic waves orhigh frequency waves, and particularly by an acoustic emission (AE)method, and as a result, stable continuous production is enabled. Thedamage monitoring is preferably used in combination with the tension ofthe constitution (i) described above, and can be used in furthercombination with any one of the (ii) airbrush below the sieve and the(iii) tapping material below the sieve.

(AE Method; Acoustic Emission Method)

The AE method is capable of detection of the initial signs of crackgeneration, and may also be used for the monitoring of crack generationduring operation or for the monitoring of the progress of cracks. The AE(acoustic emission) waves are detected by using an AE sensor that isattached directly to a propagation medium. Inside the sensor, apiezoelectric element is fixed so as to convert the AE waves that havearrived to an electric signal. This signal is amplified by about 40 dB(100 times) at a built-in amplifier in order to improve the S/N(signal/noise) ratio, and is further amplified, optionally, by about 20dB (10 times) at an externally attached amplifier. Subsequently, theamplified signal is inputted to the AE measuring apparatus to beanalyzed.

AE (acoustic emission) is defined as a “phenomenon in which when amaterial is deformed or cracks are generated, the material emits thestrain energy accumulated within the material, as an elastic wave.” Amethod of evaluating the course of destruction of a material bydetecting this elastic wave with a transducer, that is, an AE sensor,installed at the surface of the material, and carrying out signalprocessing, is an AE method. The AE signal that is detected is usuallyin a frequency range of several kHz to several MHz. For example, inmetal materials, the AE generated therefrom frequently transmit signalshaving components in the frequency region of mainly 100 kHz to 1,000kHz.

The AE sensor that is used to detect signals, is generally equipped witha piezoelectric element such as PZT (lead zirconate titanate) and isclosely attached to the material surface by means of an adhesive or anacoustic coupler such as a silicone grease, to thereby detect AEsignals. The acoustic emission (AE) signals can be categorized as aburst type waveform, and the observed AE signals can be categorized intotwo kinds having different properties. One of them is an AE waveformthat is called burst type, in which the waveform has a sharp rise and isattenuated, and the other is a continuous AE waveform with a relativelyhigh frequency. Since the burst type AE waveform is transmitted as aresult of crack propagation or transformation occurring mainly insolids, the burst type AE waveform is detected in accordance with theprogress of destruction.

(c) Place of Classification Step

The classification step of the present invention is provided beforeand/or after the surface crosslinking step, and preferably, theclassification step is carried out before and after the surfacecrosslinking step. Furthermore, it is preferable that the classificationstep be provided in least two places after the surface crosslinkingstep, and more preferably, the classification step is provided in two ormore places after the surface crosslinking step and is also provided inone or more places before the surface crosslinking step. Particularly,preferably, the classification step is provided in two or more placesafter the surface crosslinking step and also in two or more placesbefore the surface crosslinking step. When the classification step isprovided in two or more places, particularly in two places, aftersurface crosslinking, classification is first carried out after surfacecrosslinking, subsequently classification of fine powder (particularly,a fine powder having a particle size of 200 μm or less, more preferablyparticles having a size of 150 μm or less, and still more preferably 80μm to 120 μm) is carried out as a final step, and immediately afterthat, the powder is stored in a product hopper (final hopper). That is,according to the preferred embodiment, the classification step iscarried out as a final step immediately before the product is stored ina product hopper. It is preferable because this operation allows areduction in the fine powder or powder dust, and an enhancement ofliquid permeability (GBP or SFC).

Furthermore, when the distance of transport (for example, pneumatictransport) from the classification step to the subsequent step is long,it is not preferable because fine powder or powder dust may be generatedagain by damage to the product during transport. Therefore, the distancefrom the final classification step, particularly the fine powderclassification, to the product hopper is preferably set to 0 m to 100 mor less, and preferably, in the following sequence, 50 m or less, 25 mor less, 10 m or less, and 5 m or less.

(d) Removal of Electricity in Classification

In the present invention, it is preferable to implement removal ofelectricity in at least one or more of the classification steps of theconstitutions (i) to (v) described above (preferably, the sieveclassification step). When removal of electricity is achieved in theclassification steps, the physical property of the surface crosslinkedwater absorbent resin, particularly liquid permeability (for example,SFC), is improved. Such an effect is markedly exhibited in theproduction of a water absorbent resin having high liquid permeability orindustrial continuous production, particularly when a water absorbentresin having an SFC of 10[×10⁻⁷ cm³·s·g⁻¹] or more is produced orcontinuous production at a rate of 1 [t/hr] or more is continued for 24hours or longer, rather than small scale production at a laboratorylevel. Classification by removal of electricity is described in PatentLiterature 30 described above (WO 2010/032694 A).

(Method for Removal of Electricity)

In the present invention, removal of electricity may be carried out forat least one of the “classifying apparatus”, the “water absorbent resinpowder” and the “sieve.” Since these three are brought into contact witheach other in a classification step, any one of them may be removed ofelectricity. It is preferable to remove of electricity of theclassifying apparatus and/or the sieve itself.

As the removal of electricity method, for example, the following methods(A) to (C) can be applied, but the present invention is not intended tobe limited to these.

(A) Removing of electricity brush: removal of electricity from the sievesurface where static electricity has been generated.

(B) Ionized gas stream (ion generating brush): removal of electricity byapplying a high voltage and thereby generating ions.

(C) Ground connection (earth): removal of electricity of staticelectricity generated in a rotating object, a rotating shaft, a rotatingbody and an apparatus

In the case of using a removing of electricity brush of the item (A)described above, a self-discharge method of providing a small gapbetween the removing of electricity brush and a charged object may beemployed, or a ground leakage method of removing electric charge bybringing a grounded (earthed) removing of electricity brush into contactwith a charged object, and removing the accumulated static electricityas a leak current may also be employed. As specific examples of such aremoving of electricity brush, brushes produced from a stainless steelfiber, a carbon fiber, an amorphous fiber, a chemical fiber, a plantfiber, animal hair, and the like are preferred, and the wire diameter isusually preferably 1 μm to 100 μm, and more preferably 5 μm to 20 μm.Further, the wire length is usually preferably 1 mm to 100 mm, andstainless steel extra-fine processing is more preferred.

In the case of using the ionized gas stream (ion generating brush) ofthe item (B) described above, an example of the ion generating brushthat can be used includes a static eliminator (ionizer). In such aremoval of electricity method, ions are produced in air or in anothergas, and the electrification charge is neutralized by these ions. Forthat reason, a removing of electricity apparatus is also called anionizer. Specifically, the amount of electric charge and theelectrification charge of a classifying apparatus or a water absorbentresin may be measured by using a static eliminator (ionizer), and anelectrically neutral state may be attained by applying an oppositecharge to the plus (+) charge or the negative (−) charge. At this time,a balance may be achieved between the optimal removal of electricity inaccordance with the condition of electrification of a charged object andthe control of ion balance. The amount of electric charge of the chargedobject may be determined by measuring the ion current by using an ioncurrent detection circuit mounted in the controller. As such, the method(B) of completely deactivating static electricity by neutralizing thecharge with a charge of reverse polarity is a preferred method for waterabsorbent resins.

In the case of using the ground connection of the item (C) describedabove, a method of removing of electricity by electrically connecting abuilding or a stand, on which the classifying apparatus is installed, toan earth with the ground resistance value described below, bringing acharged object into contact with the apparatus, and removing theaccumulated static electricity as a leak current, may be used. Thismethod is simple and easy, and is highly effective because theclassifying apparatus as a whole works as a removing of electricityapparatus. Thus, it is one of the preferred methods for water absorbentresins.

The leak current removed at the time of such removal of electricitypreferably flows to the ground through the ground connection (earth)with a ground resistance value described below. That is, in theclassification step of the present invention, it is preferable thatremoval of electricity be achieved by using a ground connection with aground resistance value described below.

(Ground Resistance)

Ground resistance means a resistance value against a current that flowsfrom an earth electrode buried in the soil for a ground connection tothe ground. As the measurement method, measurement may be made by usinga commercially available ground resistance meter. A preferred range ofthe ground resistance value is preferably 100Ω or less, more preferably10Ω or less, and still more preferably 5Ω or less, from the viewpoint ofthe classification step. There are no particular limitations on thelower limit of the ground resistance value, and a smaller value is morepreferred. However, the ground resistance value is usually 1Ω orgreater.

(e) Classifying Apparatus

The present invention is preferably constituted such that theclassifying apparatuses described below are further used in at least oneor more classification steps (preferably, sieve classification steps) ofthe constitutions (i) to (v) described above.

(Classifying Mesh)

In the present invention, a water absorbent resin powder is classifiedby using a classifying mesh. Examples of the classifying mesh includevarious standard sieves of JIS, ASTM and TYLER. These sieves may beplate sieves or may be mesh sieves. The shape of the mesh sieve isappropriately selected by making reference to JIS Z8801-1 (2000) or thelike. The mesh opening size of the standard sieves is preferably in therange of 10 μm to 100 mm, and still more preferably in the range of 20μm to 10 mm, and it is preferable to use one kind or two or more kindsof sieves, particularly metal sieves.

Furthermore, the area of the sieve mesh (area of the sieve mesh surface)is preferably 1 to 10 m²/sheet, and more preferably 1.5 to 7 m²/sheet,from the viewpoint of classification efficiency.

The sieve classification may be carried out such that only the upperpart may be classified, or only the lower part may be classified.Preferably, however, it is preferable to classify the upper limit andthe lower limit simultaneously. That is, it is preferable tosimultaneously use plural sieves, and still more preferably, sieves ofat least three kinds of mesh opening sizes are used in view of physicalproperty improvement. As such a method, it is preferable to useintermediate sieves or upper sieves, in addition to the predeterminedsieves of the upper position and the lower position. A suitable sieveis, for example, a sieve having a mesh opening size of 850 μm to 1,000μm, 710 μm to 850 μm, or 600 μm to 710 μm, as the upper limit, and asieve having a mesh opening size of 150 μm to 225 μm as the lower limit.Still more preferably, sieves may be appropriately added in the middleor in the upper part.

(Classifying Apparatus)

The classifying apparatus used in the present invention is notparticularly limited as long as it has a sieve mesh surface, andexamples include apparatuses that are categorized into vibrating screensand sifters. Examples of the vibrating screens include a tilted type, aLow-head type, Hum-mer, Rhewum, Ty-Rock, Gyrex, Eliptex, and the like,and examples of the sifters include a reciprocating type, Exolon-grader,Traversator-sieb, Sauer-meyer, Gyratory, Gyrosifter, Ro-tex, and thelike. These are minutely categorized based on the shape of motion of themesh surface (circular, elliptical, linear, arc, para-oval, spiral, orhelical), mode of vibration (free vibration or forced vibration),driving method (eccentric shaft, unbalanced weight, electromagnetic, orimpact), tilt of the mesh surface (horizontal or tilted), method ofinstallation (floor type or suspended type), or the like. Among them, itis preferable that the metal sieve mesh trace a three-dimensional motiontrajectory composed of eccentric tilt, radial tilt (tilt of the sievemesh that disperses the material from the center to the periphery) ortangential tilt (tilt of the sieve mesh that controls the dischargespeed over the mesh).

Particularly, in view of the effect of the present invention, aclassifying apparatus which moves the sieve mesh surface in a helicalform by the combination of radial tilt or tangential tilt, such as anoscillatory type apparatus (tumbler-screening machine), is preferred.

(Classification vibration)

There are no limitations on the sieve classifying apparatus that isappropriate for the classification method according to the presentinvention; however, preferably, it is preferable to use a planeclassification method, and a tumble type sieve classifying apparatus isparticularly preferred. This sieve classifying apparatus is typicallyvibrated in order to support classification. The vibration is preferablycarried out to the extent that the product to be classified is guided toa spiral form (helical form) on the sieve. Such forcible vibrationtypically has an amplitude of vibration of 0.7 mm to 40 mm, andpreferably 1.5 mm to 25 mm, and a frequency of vibration of 60 rpm to6,000 rpm, and preferably 100 rpm to 600 rpm.

(Guide)

In the present invention, it is also preferable that the sieve of theclassifying apparatus have a guide for the water absorbent resin powder.By installing such a guide, classification can be carried out moreefficiently. Such a guide apparatus has a function of guiding a waterabsorbent resin powder to the center of the sieve or the like, and thelength is determined to be about 5% to 40% of the diameter.

(Material and Surface Roughness)

The material of the sieve apparatus is not particularly limited, and isappropriately selected from resins, metals and the like. However, ametallic sieve, where a contact surface with the water absorbent resinis also metallic, is preferred, and a stainless steel sieve isparticularly preferred, as compared with a resin-coated sieve mentionedas an example in JP 11-156299 A. In this case, the effect of the presentinvention is more effectively exhibited. When stainless steel ismirror-surface finished, the physical property is even further improved.Examples of stainless steel include SUS304, SUS316, SUS316L, and thelike. Magnestain is also suitably used.

From the viewpoint of an enhancement of the physical property, theadhesion of water absorbent resin fine particles to the inner surfacesof the sieve apparatus, and the like, it is preferable that the innersurface of the sieve apparatus (metal sieve mesh) used in theclassification step in the present invention have the surface roughness(Rz) defined by JIS B 0601-2001 controlled to be 800 nm or less. Thatis, preferably, the material of the metal sieve mesh is SUS304 orSUS316, and the surface roughness (Rz) of the inner surface of the metalsieve mesh is 800 nm or less. The inner surface is flattened to have asurface roughness (Rz) of preferably 150 nm or less, still morepreferably 100 nm or less, and particularly preferably 50 nm or less.Meanwhile, the surface roughness (Rz) means the maximum value of themaximum height (μm) of the surface asperity. The lower limit of thesurface roughness (Rz) is 0 nm, but there is no big difference even atabout 10 nm, and it is still sufficient even with a surface roughness of10 nm or 20 nm. Another surface roughness (Ra) is also defined by JIS B0601-2001. Ra is preferably 100 nm or less, more preferably 50 nm orless, and particularly preferably 5 nm or less. The lower limit of thesurface roughness (Ra) is 0 nm, but there is no big difference even atabout 1 nm. Such surface roughness can be measured with a probe typesurface roughness meter according to JIS B 0651-2001. Further, suchsurface roughness can be measured with a light wave interference typesurface roughness meter according to JIS B 0652-2002.

(f) Conditions for Classification

The present invention is preferably achieved under the conditionsdescribed below in at least one or more classification steps(preferably, sieve classifying steps) of the constitutions (i) to (v)described above, and particularly preferably, the sieves are heated orkept warm to a predetermined temperature. Furthermore, preferably, theconditions for classification include reduced pressure and ventilationof an gas stream.

(Heating Temperature)

According to the present invention, preferably, a classifying apparatusis used in a heated state and/or in a heat-retained state, in additionto removal of electricity. Furthermore, preferably, the water absorbentresin is also used in a state of being heated to a predeterminedtemperature.

Heating in the present invention refers to active provision of heat.Therefore, a heated state includes a case in which the classifyingapparatus is heated to increase the temperature to a certain temperaturein the early phase, and then heat is no longer supplied; a case in whichheat is supplied to the classifying apparatus not only in the earlyphase but also steadily, and the like. Meanwhile, keeping warm meansmaking it difficult to liberate heat without supplying heat, that is,making it difficult for the temperature to decrease.

In order to bring a classifying apparatus to a heated state and/or aheat-retained state, the temperature of the atmosphere in which theclassifying apparatus is placed may be increased, or the like. Apreferred classifying apparatus is a dry classifying apparatus equippedwith a heating unit and/or a heat retention unit.

Such a classifying apparatus (temperature of the sieve used) ispreferably used at a temperature in the range of 40° C. or higher, orfurther 40° C. to 80° C. More preferably, the classifying apparatus isused at a temperature in the range of 45° C. to 60° C. When thetemperature is 40° C. or higher, deterioration of the physical propertyis prevented, and when the temperature is lower than 100° C. or 80° C.,the economic inefficiency caused by high temperature can be prevented,and the adverse effect on the classification efficiency can beprevented.

It is preferable that the classifying apparatus be used at a temperaturethat is not lower by 20° C., and more preferably not lower by 10° C.,than the temperature of the water absorbent resin powder.

Furthermore, when a water absorbent resin powder is handled, the waterabsorbent resin powder introduced to the classification step is broughtto a temperature equal to or higher than room temperature, andpreferably 40° C. or higher. For example, it is preferable to heat thewater absorbent resin powder to 40° C. to 100° C., and more preferablyto 50° C. to 80° C. In order to obtain a water absorbent resin at such atemperature, the water absorbent resin may be appropriately heated, or awater absorbent resin obtained after the heating at the drying step orthe surface crosslinking step may be kept warm.

(Reduced Pressure)

It is preferable to carryout the classification step under reducedpressure in order to enhance the physical property after surfacecrosslinking. The term “reduced pressure” means a state in which the airpressure is lower than the atmospheric pressure, and the pressuredifference between the air pressure and the atmospheric pressure isexpressed as a positive (plus) value. For example, when the atmosphericpressure is standard atmospheric pressure (101.3 kPa), the phrase “thedegree of pressure reduction is 10 kPa,” means that the air pressure is91.3 kPa.

According to the present invention, from the viewpoint of classificationefficiency, the lower limit of the degree of pressure reduction ispreferably greater than 0 kPa, more preferably 0.01 kPa or greater, andstill more preferably 0.05 kPa or greater. Furthermore, from theviewpoint of suppressing uplift of the powder in the system and loweringthe cost for the exhaust apparatus, the upper limit of the degree ofpressure reduction is preferably 10 kPa or less, more preferably 8 kPaor less, still more preferably 5 kPa or less, and particularlypreferably 2 kPa or less. A preferred value range of the degree ofpressure reduction can be arbitrarily selected to be between the lowerlimit and the upper limit described above.

(Gas Stream)

In the classification step, it is preferable that a gas stream passesthrough, and preferably, a gas stream, and particularly preferably airis passed through over the water absorbent resin powder. Preferably,countercurrent gas streams are ventilated above and below the sieve mesh(metal sieve mesh). The amount of this gas is typically 0.1 to 10[m³/hr], preferably 0.5 to 5 [m³/hr], and particularly preferably 1 to 3[m³/hr], per 1 m² of the sieve area, and at that time, the gas volume ismeasured under standard conditions (for example, under the conditions of25° C. and 1 bar). Particularly preferably, the gas stream is heated,before being introduced into the sieve classifying apparatus, typicallyto 40° C. or higher, preferably 50° C. or higher, more preferably 60° C.or higher, still more preferably 65° C. or higher, and particularlypreferably 70° C. or higher. The temperature of the gas stream isusually 120° C. or lower, preferably 110° C. or lower, more preferably100° C. or lower, still more preferably 90° C. or lower, andparticularly preferably 80° C. or lower.

The dew point of the gas stream is preferably 15° C. or lower, and morepreferably 10° C. or lower. There are no particular limitations on thelower limit of the dew point, but in consideration of cost performance,the lower limit of the dew point is preferably −10° C. or higher, andmore preferably about −5° C.

(Atmospheric Dew Point)

The dew point of the atmosphere (air) in which the classification stepis carried out is preferably 15° C. or lower, more preferably 10° C. orlower, still more preferably 5° C. or lower, and particularly preferably0° C. or lower. There are no particular limitations on the lower limitof the dew point, but in consideration of cost performance, the lowerlimit of the dew point is preferably −10° C. or higher, and morepreferably −5° C. or higher. Furthermore, the temperature of the gas ispreferably 10° C. to 40° C., and more preferably 15° C. to 35° C.

As the method of controlling the dew point, the gas, preferably air, maybe appropriately dried, and examples thereof include a method of using amembrane dryer, a method of using a cooling adsorption dryer, a methodof using a diaphragm dryer, and methods of using these dryers incombination. In the case of using an adsorption dryer, the adsorptiondryer may be a heating regeneration type, a non-heating regenerationtype, or a non-regeneration type.

(Number of Apparatuses)

In the production method of the present invention, from the viewpointsof an enhancement and stabilization of the physical property of thewater absorbent resin, the polymerization step is carried out bycontinuous belt polymerization or continuous kneader polymerization, andit is preferable that in a series of the polymerization steps, pluralclassification steps be carried out in parallel.

Here, the term “a series” in the present invention means one serieswhere the steps are carried out from the stage of raw materials(monomers) to the stages of obtaining a water-containing gel-likecrosslinked polymer, a water absorbent resin powder, a water absorbentresin, and a final product with progression of processes. When thesystem is branched into two systems, the case is called “two series”.Therefore, the term “two or more series” refers to an embodiment inwhich in the same step, two or more apparatuses are disposed inparallel, and those apparatuses are operated simultaneously oralternately. Furthermore, without being limited to the classificationstep only, it is preferable that the pulverization step, the surfacecrosslinking step and the like coming after the drying step be carriedout in two series. That is, in a series of the polymerization stepdescribed above, it is preferable that the classification step becarried out in two series, and it is most preferable that all the stepssuch as the pulverization step and the surface crosslinking step becarried out in two or more series.

(2-6) Surface Crosslinking Step

The present step is a step of crosslinking the vicinity of the surfaceof the water absorbent resin powder obtained in the pulverization stepor the classification step described above by using a surfacecrosslinking agent (surface crosslinking reaction), for an enhancementof the absorption performance, and through the surface crosslinking, awater absorbent resin which undergoes less coloration and has a higherdegree of whiteness is obtained.

There are no particular limitations on the surface crosslinking agentthat can be used in the present invention, but various organic orinorganic surface crosslinking agents (ionic bonding surfacecrosslinking agents) may be used. Among them, organic surfacecrosslinking agents are preferred, and it is more preferable to use anorganic surface crosslinking agent and an ionic bonding surfacecrosslinking agent (ionic crosslinking agent) in combination.

Specific examples of the organic surface crosslinking agents includepolyhydric alcohol compounds, epoxy compounds, polyvalent aminecompounds or condensates thereof with haloepoxy compounds, oxazolinecompounds, (mono-, di- or poly-)oxazolidinone compounds, and alkylenecarbonate compounds, and particularly, dehydration ester-reactivecrosslinking agents containing polyhydric alcohol compounds, alkylenecarbonate compounds and oxazolidinone compounds, which require reactionsat high temperatures, can be used. More specifically, the compoundslisted as examples in U.S. Pat. No. 6,228,930, U.S. Pat. No. 6,071,976,U.S. Pat. No. 6,254,990, and the like can be used. Examples includepolyhydric alcohol compounds such as mono-, di-, tri-, tetra- orpropylene glycol, 1,3-propanediol, glycerin, 1,4-butanediol,1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, and sorbitol; epoxycompounds such as ethylene glycol diglycidyl ether and glycidol;alkylene carbonate compounds such as ethylene carbonate; oxetanecompounds; and cyclic urea compounds such as 2-imidazolidinone and thelike. The use amount of the organic surface crosslinking agent isappropriately determined in the range of preferably 0.001 parts to 10parts by weight, and more preferably 0.01 parts to 5 parts by weight,relative to 100 parts by weight of the water absorbent resin powder.

Furthermore, it is preferable to use water as a solvent at the time ofmixing the water absorbent resin and the surface crosslinking agent. Theuse amount of water described above is appropriately determined in therange of preferably 0.5 parts to 20 parts by weight, and more preferably0.5 parts to 10 parts by weight, relative to 100 parts by weight of thewater absorbent resin powder. Further, a hydrophilic organic solvent maybe optionally used in combination, in addition to water. The use amountthereof is appropriately determined in the range of preferably 0 partsto 10 parts by weight, and more preferably 0 parts to 5 parts by weight,relative to 100 parts by weight of the water absorbent resin powder.

Furthermore, at the time of mixing a surface crosslinking agentsolution, a water-insoluble fine particle powder (water-insolubleparticle) or a surfactant may be incorporated to the extent that theeffect of the present invention is not impaired. In regard to the type,use amount, or the like of the fine particle powder or the surfactant,examples are given in U.S. Pat. No. 7,473,739 and the like. However, theuse amount is appropriately determined in the range of preferably 0parts to 10 parts by weight, more preferably 0 parts to 5 parts byweight, and still more preferably 0 parts to 1 part by weight, relativeto 100 parts by weight of the water absorbent resin powder.

In the present step, after the water absorbent resin powder and thesurface crosslinking agent are mixed, the mixture is preferablysubjected to heating treatment and subsequently to optionally coolingtreatment. The heating temperature at the time of the heating treatmentdescribed above is preferably 70° C. to 300° C., more preferably 120° C.to 250° C., and still more preferably 150° C. to 250° C. If thetreatment temperature described above is lower than 70° C., the heatingtreatment time increases, causing a decrease in productivity, and also auniform surface crosslinked layer cannot be formed, which is notpreferable. Furthermore, if the treatment temperature is higher than300° C., the water absorbent resin powder is deteriorated, and it is notpreferable. Furthermore, the heating time at the time of the heatingtreatment described above is preferably in the range of 1 minute to 2hours. The heating treatment described above can be carried out in aconventional dryer or a conventional heating furnace. Meanwhile, thesurface crosslinking methods disclosed in EP 0349240 A, EP 0605150 A, EP0450923 A, EP 0812873 A, EP 0450924 A, EP 0668080 A, JP 7-242709 A, JP7-224304 A, U.S. Pat. No. 5,409,771, U.S. Pat. No. 5,597,873, U.S. Pat.No. 5,385,983, U.S. Pat. No. 5,610,220, U.S. Pat. No. 5,633,316, U.S.Pat. No. 5,674,633, U.S. Pat. No. 5,462,972, WO 99/42494 A, WO 99/43720A, WO 99/42496 A, and the like can be preferably applied to the presentinvention.

(Inorganic Surface Crosslinking Agent)

According to the present invention, an inorganic surface crosslinkingagent (ionic bonding surface crosslinking agent) can be used other thanthe organic surface crosslinking agent described above, for the purposeof enhancing the physical property such as liquid permeability. Thereare no particular limitations on the inorganic surface crosslinkingagent used, but examples include polyvalent metal salts (organic saltsor inorganic salts) having a valence of 2 or higher, and preferably avalence of 3 or 4, or hydroxides. Specific examples of the polyvalentmetals include aluminum, zirconium and the like, and aluminum lactateand aluminum sulfate are preferably used. The polyvalent metal salts arepreferably used in a solution state, and are more preferably in the formof an aqueous solution, from the viewpoints of handleability and themiscibility with the water absorbent resin powder. The amount ofaddition of the inorganic surface crosslinking agent is such that theoptimum amount may vary with the type or particle size of the waterabsorbent resin, but usually, the amount of addition is in the range ofgreater than 0 and equal to or less than 10 parts by weight, preferably0.001 parts to 5 parts by weight, and still more preferably 0.002 partsto 3 parts by weight, relative to 100 parts by weight of the solidcontent of the water absorbent resin powder.

These inorganic surface crosslinking agents are used simultaneously withor separately from the organic surface crosslinking agents. Preferably,from the viewpoint of water absorption performance (particularly, highliquid permeability), it is preferable to add an inorganic surfacecrosslinking agent to the water absorbent resin powder after the surfacecrosslinking by means of an organic surface crosslinking agent (forexample, at the time of a cooling treatment).

Meanwhile, surface crosslinking by means of polyvalent metals isdescribed in WO 2008/09843, U.S. Pat. No. 7,157,141, U.S. Pat. No.6,605,673, U.S. Pat. No. 6,620,889, US 2005/0288182 A, US 2005/0070671A, US 2007/0106013 A, and US 2006/0073969 A, or the like.

(Cooling Treatment)

A cooling treatment is optionally carried out, for the purpose ofstopping, controlling or the like the surface crosslinking reaction,before a water absorbent resin powder obtained by being heated in thesurface crosslinking step to have the surface vicinity crosslinked isfed to a subsequent step (for example, a classification step (particlesize regulation step)). There are no particular limitations on thecooling apparatus used in this cooling treatment, but for example, apaddle dryer; a biaxial stirred dryer in which cooling water iscirculated in the inner wall and inside other heat transfer surfaces; agroove type stirred dryer; or the like can be used. The temperature ofthe water absorbent resin powder obtained after the cooling treatmentcan be adjusted to below the heating temperature, that is, equal to orhigher than 25° C. and lower than 80° C., and preferably can be adjustedto equal to or higher than 30° C. and equal to or lower than 60° C.However, in the case of carrying out a classification step (particlesize regulation step) after the cooling treatment, it is preferable tocarry out a cooling treatment for the water absorbent resin powder suchthat the conditions described above in the section (f) Conditions forclassification are satisfied.

Meanwhile, in the surface crosslinking step, there are occasions inwhich the surface crosslinking of a water absorbent resin powder iscarried out at room temperature. In this case, since the water absorbentresin powder obtained by surface crosslinking is not heated, thiscooling treatment may not be carried out.

Furthermore, for the purpose of enhancing the physical property such asliquid permeability, a polyamine polymer may also be used simultaneouslyor separately, other than the organic surface crosslinking agent and theinorganic surface crosslinking agent. The polyamine polymer is such thata polyamine polymer having a weight average molecular weight of about5,000 to 1,000,000 is particularly preferred, and examples thereof arelisted in U.S. Pat. No. 7,098,284, WO 2006/082188 A, WO 2006/082189 A,WO 2006/082197 A, WO 2006/111402 A, WO 2006/111403 A, and WO 2006/111404A, and the like.

A water absorbent resin obtained by surface treating with the organicsurface crosslinking agent, inorganic surface crosslinking agent andadditive described above, preferably has at least one kind of awater-insoluble particle, a polyamine polymer and a polyvalent metal onthe surface. A water absorbent resin in this form has excellent physicalproperty (particularly, liquid permeability).

(2-7) Fine Powder Recycling Step (Fine Powder Collection Step andGranulation Step)

A fine powder recycling step according to the present invention is aprocess of separating a fine powder (particularly, a fine powdercontaining particles having a particle size of less than 150 μm as amain component, and particularly at a proportion of 70% by weight ormore) obtained by drying and optionally by pulverizing and classifying,and then recycling the fine powder to the polymerization step or thedrying step directly or after hydration of the fine powder. For example,the methods described in US 2006/247351 A, U.S. Pat. No. 6,228,930 andthe like, can be applied. In the present invention, it is preferable tofurther include the fine powder recycling step after the classificationstep.

When a recycled fine powder is incorporated, the particle size can becontrolled, and at the same time, a high solid content concentration,which is essential in the present invention, can be easily achieved byaddition of the water absorbent resin powder. Furthermore, detachment ofthe water absorbent resin obtained after drying from the drying belt canbe facilitated by the addition of a fine powder, and therefore, it ispreferable.

(2-8) Deferrization Step

In the present invention, there are occasions in which metal wires areincorporated, such as in the case of using the removing of electricitybrush described above, and thus, preferably a deferrization step, andmore preferably a deferrization step using a magnet, is included afterthe classification step. When the deferrization step is carried out,metal components that are present in the water absorbent resin powdercan be removed. A permanent magnet may be used for deferrization, andmetal originating from a sieve, a brush, or the like may be removed byallowing a water absorbent resin powder that continuously flows, to passthrough between magnets.

(2-9) Transport Step

In regard to the method of transporting a water absorbent resin beforeand after the classification step, and particularly preferably betweenthe classification step and the surface crosslinking step, variousmethods can be used, but preferably, pneumatic transport is used. Forthe pneumatic transport, it is preferable to use dry air from theviewpoint that the excellent physical property of the water absorbentresin is stably retained. The upper limit of the dew point of the dryair is usually 20° C. or lower, preferably −5° C. or lower, morepreferably −10° C. or lower, still more preferably −12° C. or lower, andparticularly preferably −15° C. or lower. Furthermore, the lower limitof the dew point is usually −100° C. or higher, and preferably −70° C.or higher, and a dew point of about −50° C. is sufficient. Furthermore,the temperature of the dry air is preferably 10° C. to 40° C., and morepreferably 15° C. to 35° C. The surface roughness (Rz) of the innersurface of the pneumatic transport pipe is in the same range as that ofthe surface roughness (Rz) of the inner surface of the sieve apparatusdescribed above.

A heated gas (air) may also be used other than a dry gas (air). In thiscase, the heating method is not particularly limited, but the gas (air)may be directly heated by using a heat source, or the transport pipes orapparatuses are heated and thereby the gas (air) passing therethroughmay be indirectly heated. The lower limit of the temperature of thisheated gas (air) is preferably 20° C. or higher, and more preferably 30°C. or higher. Furthermore, the upper limit of the temperature of theheated gas (air) is preferably lower than 70° C., and more preferablylower than 50° C.

As the method of controlling the dew point, the gas (preferably, air)may be appropriately dried. Specific examples include a method of usinga membrane dryer, a method of using a cooling adsorption dryer, a methodof using a diaphragm dryer, and methods of using these dryers incombination. In the case of using an adsorption dryer, the adsorptiondryer may be a heating regeneration type, or a non-heating regenerationtype.

(2-10) Other Steps

In addition to the continuous steps described above, the fine powderrecycling step described above, a granulation step, a fine powderremoval step, and the like may be optionally provided. Furthermore, forthe purpose of a color stability effect over time, prevention of geldeterioration, or the like, the additive described above may beoptionally used, in some or all of the various steps described above.Furthermore, the production method of the present invention preferablyincludes a fine powder recycling step. Further, preferably, theproduction method further includes one or two or more steps such as atransport step, a storage step, a packing step, steps for adding otheradditive (a fine particle, a deodorizer, an antibacterial agent, and thelike), and the like.

[3] PHYSICAL PROPERTIES OF WATER ABSORBENT RESIN

The water absorbent resin of the present invention contains apolyacrylic acid (salt)-type water absorbent resin as a main component,and is obtained by the polymerization method described above, surfacecrosslinking method or the like, when a use of the water absorbent resinin sanitary products, particularly paper diapers, is intended.Furthermore, for the water absorbent resin after the surfacecrosslinking step, it is preferable to control at least one or morephysical properties among the various physical properties discussed inthe following sections (3-1) to (3-7), and it is preferable to controltwo or more, particularly three or more, physical properties includingthe AAP. In the present invention, the term “water absorbent resin afterthe surface crosslinking step” means a water absorbent resin as a finalproduct. For this reason, when a classification step is carried outafter the surface crosslinking step, the water absorbent resin after thesurface crosslinking step means a water absorbent resin as a finalproduct after the classification step. When the water absorbent resinsatisfies various physical properties such as described below, the waterabsorbent resin can exhibit sufficient performance even in highconcentration diapers having a water absorbent resin concentration of40% by weight or more.

(3-1) CRC (Absorption Capacity without Load)

The CRC (absorption capacity without load) of the water absorbent resinobtained by the present invention is preferably 10 [g/g] or greater,more preferably 20 [g/g] or greater, still more preferably 25 [g/g] orgreater, and particularly preferably 27 [g/g] or greater. The upperlimit of the CRC is not particularly limited, but the upper limit ispreferably 50 [g/g] or less, more preferably 45 [g/g] or less, and stillmore preferably 40 [g/g] or less. If the CRC is less than 10 [g/g], theabsorption amount of the water absorbent resin is low, and there is arisk that the water absorbent resin may not be suitable for the use inthe absorbent materials in sanitary products such as paper diapers.Furthermore, if the CRC described above exceeds 50 [g/g], when such awater absorbent resin is used in absorbent materials, there is a riskthat sanitary products having an excellent rate of liquid uptake may notbe obtained, which is not preferable. Meanwhile, the CRC can beappropriately controlled by the internal crosslinking agent, surfacecrosslinking agent described above, or the like.

(3-2) AAP (Absorption Capacity Under Load)

As to the AAP (absorption capacity under load) of the water absorbentresin obtained by the present invention, for the purpose of preventionof leakage into paper diapers, which is achieved by the drying describedabove, the AAP under a pressure of 4.83 kPa (0.7 psi) is preferably 20[g/g] or greater, more preferably 22 [g/g] or greater, and still morepreferably 24 [g/g] or greater. The upper limit of the AAP is notparticularly limited, but in view of the balance with other physicalproperties, the upper limit is preferably 40 [g/g] or less. When the AAPdescribed above is less than 20 [g/g], if such a water absorbent resinis used in an absorbent material, there is a risk that a sanitaryproduct which exhibits less return of liquid (usually, also referred toas “re-wet”) when pressure is applied to the absorbent material, may notbe obtained, and this is not preferable. Meanwhile, the AAP can beappropriately controlled by the surface crosslinking agent, particlesize described above, or the like.

(3-3) SFC (Saline Flow Conductivity)

As to the SFC (saline flow conductivity) of the water absorbent resinobtained by the present invention, for the purpose of prevention ofleakage into paper diapers, which is achieved by the drying describedabove, the SFC which is a liquid permeability characteristic of a liquidunder pressure is preferably 1[×10⁻⁷ cm³·s·g⁻¹] or greater, morepreferably 10[×10⁻⁷ cm³·s·g⁻¹] or greater, still more preferably30[×10⁻⁷ cm³·s·g¹] or greater, particularly preferably 70[×10⁻⁷cm³·s·g⁻¹] or greater, and most preferably 110[×10⁻⁷ cm³·s·g⁻¹] orgreater. The upper limit of the SFC is not particularly limited, but inview of the balance with other physical properties, the upper limit ispreferably 3,000[×10⁻⁷ cm³·s·g⁻¹] or less, and more preferably2,000[×10⁻⁷ cm³·s·g⁻¹] or less. When the SFC exceeds 3,000[×10⁻⁷cm³·s·g⁻¹] or greater, if such a water absorbent resin is used in anabsorbent material, there is a risk that liquid leakage in the absorbentmaterial may occur, and it is not preferable. Meanwhile, the SFC can beappropriately controlled by the drying method described above or thelike.

(3-4) Ext (Extractables)

The Ext (extractables) of the water absorbent resin obtained by thepresent invention is preferably 35% by weight or less, more preferably25% by weight or less, still more preferably 15% by weight or less, andparticularly preferably 10% by weight or less. If the Ext is greaterthan 35% by weight, the gel strength of the water absorbent resinobtained is weak, and there is a risk that liquid permeability maydeteriorate. Furthermore, when such a water absorbent resin is used inan absorbent material, there is a risk that a water absorbent resinwhich exhibits less return of liquid (re-wet) when pressure is appliedto the absorbent material, may not be obtained, and this is notpreferable. Meanwhile, the Ext can be appropriately controlled by theinternal crosslinking agent described above or the like.

(3-5) Residual Monomers

The amount of residual monomers of the water absorbent resin obtained bythe present invention is controlled, from the viewpoint of safety,preferably to 0 ppm to 500 ppm, more preferably 0 ppm to 400 ppm, stillmore preferably 0 ppm to 300 ppm, and particularly preferably 0 ppm to200 ppm. Meanwhile, the amount of residual monomers can be appropriatelycontrolled by the polymerization method described above or the like.

(3-6) Initial Color Tone

The water absorbent resin obtained by the present invention hasexcellent initial color tone. That is, the color tone of the waterabsorbent resin immediately after production (initial color tone), whichmay be obtained by the present invention, exhibits the following values.Meanwhile, the initial color tone refers to the color tone immediatelyafter production, but is generally considered as the color tone measuredbefore factory shipment. Furthermore, for example, if the waterabsorbent resin is stored in an atmosphere at below 30° C. and at arelative humidity of 50% RH, the initial color tone is the valuemeasured within one year after production. Specifically, with respect toHunter's Lab color scale, the L value (lightness) is preferably 85 orgreater, more preferably 87 or greater, and still more preferably 89 orgreater. Furthermore, the b value is preferably −5 to 10, morepreferably −5 to 9, still more preferably −4 to 8, and particularlypreferably −1 to 7. Further, the a value is −2 to 2, at least −1 to 1,preferably −0.5 to 1, and particularly preferably 0 to 1. Furthermore,in another color scale, the YI (yellow index) value is preferably 10 orless, more preferably 8 or less, and particularly preferably 6 or less.As another color scale, the WB (white balance) value is preferably 70 orgreater, more preferably 75 or greater, and particularly preferably 77or greater. The water absorbent resin obtained by the present inventionis also excellent in coloration over time, and exhibits a sufficientdegree of whiteness even in an acceleration test carried out under hightemperature and high humidity. Meanwhile, the initial color tone and thecoloration over time of the water absorbent resin of the presentinvention can be measured by the measurement method disclosed in WO2009/005114 A.

(3-7) Moisture Content

The moisture content of the water absorbent resin obtained by thepresent invention is, from the viewpoint of the powder characteristics(prevention of static charge, impact resistant stability, and preventionof deterioration of physical properties during transport), preferably 0%to 15% by weight, more preferably 0% to 10% by weight, and still morepreferably 0% to 3% by weight. Meanwhile, the lower limit of themoisture content is preferably 0.1% by weight or greater, morepreferably 0.5% by weight or greater, still more preferably 1% by weightor greater, and particularly preferably 1.4% by weight or greater. Theadjustment of the moisture content may be carried out by appropriatelyadjusting the heat treatment conditions at the time of the surfacecrosslinking step, or if necessary, the amount of addition of water.

Meanwhile, water in the water absorbent resin is an inhibitory factor atthe time of classification; however, in the present invention, a waterabsorbent resin having the predetermined moisture content describedabove can be more effective in the classification step, than in anabsolute dry state with a moisture content of less than 0.1% by weight.Therefore, the present invention can be preferably applied in the methodfor producing a water absorbent resin, which includes a classificationstep for the water absorbent resin having the moisture content describedabove.

[4] USE OF WATER ABSORBENT RESIN

The use of the water absorbent resin obtained by the production methodaccording to the present invention is not particularly limited, and thewater absorbent resin can be used in sanitary products such as paperdiapers, sanitary napkins, and incontinence pads; and water absorbentarticles such as agricultural and horticultural water retention agents,waste water solidifying agents, and industrial water stopping materials.

EXAMPLES

Hereinafter, the present invention will be described by way of Examplesand Comparative Examples, but the present invention is not intended tobe construed to be limited to these Examples. Furthermore, forconvenience, the unit “liter” may be indicated as “L”, and the unit “%by weight” as “wt %”. Meanwhile, unless otherwise specified, the variousphysical properties described in the claims and Examples of the waterabsorbent resin obtained by the present invention were determinedaccording to the EDANA methods and the Measurement Examples describedbelow, under the conditions of room temperature (20° C. to 25° C.) and ahumidity of 50 RH %.

1. Resin Solid Content (Solid Content)

In an aluminum cup in which the diameter of the bottom is about 50 mm,1.00 g of a water absorbent resin was weighed, and the total weight W1[g] of the sample (the water absorbent resin and the aluminum cup) wasaccurately weighed.

Subsequently, the sample was placed in an oven at an atmospherictemperature of 180° C., and thereby the water absorbent resin was dried.After a lapse of 3 hours, the sample was removed from the oven togetherwith the aluminum cup and cooled to room temperature in a desiccator.Thereafter, the total weight W2 [g] of the dried sample (the waterabsorbent resin and the aluminum cup) was weighed, and the solid content(unit: [wt %]) was calculated according to the following formula.Solid content [wt %]=100−{(W1−W2)/(weight of water absorbent resin[g])×100}  [Mathematical Formula 1]

Meanwhile, when the resin solid content of a particulatewater-containing gel-like crosslinked polymer (particulate hydrogel),the measurement was made by changing the collection amount of theparticulate hydrogel to 2 to 4 g, and the drying time to 24 hours.

2. SFC (Saline Flow Conductivity)

The SFC (saline flow conductivity) of the water absorbent resin obtainedby the present invention was measured according to the descriptions ofU.S. Pat. No. 5,669,894.

3. Other Physical Properties

The physical properties of the water absorbent resin obtained by thepresent invention such as the CRC (absorption capacity without load),the particle size distribution (see the section “PSD” described above:method described in ERT420.2-02), pH extractables (see the section “Ext”described above: method described in ERT470.2-02), and the amount ofresidual acrylic acid (see the section “Residual Monomers” describedabove: method described in ERT410.2-02) were measured according to theERT of EDANA described above, or according to US 2006/204755 A.

Example 1

The following operation was carried out according to Example 1 of PatentLiterature 30 (WO 2010/032694 A).

That is, as illustrated in FIG. 1, continuous production was achieved byusing a continuous production facility for a water absorbent resin(production capacity: 1500 [kg/hr]), which includes a polymerizationstep (static polymerization on a belt), a gel grain refining (crushing)step, a drying step, a pulverization step, a classification step(classification step 1), a surface crosslinking step (a mixing step fora surface crosslinking agent, a heating step, and a cooling step), aparticle size regulation step (classification step 2), a classificationstep 3, and transport steps that connect the respective steps.Meanwhile, the classification step 1, the surface crosslinking step, theclassification step 2 and the classification step 3 described above wereconnected by pneumatic transport (dry air having a dew point of 10° C.,or heated air at 60° C.)

Specifically, an aqueous solution (monomer concentration: 37 wt %) of apartial sodium salt of acrylic acid with a degree of neutralization of75 mol %, containing 0.06 mol % (based on the monomer) of polyethyleneglycol diacrylate (average n value (average degree of polymerization):9) as an internal crosslinking agent, was used as an aqueous monomersolution (1), and the aqueous monomer solution (1) obtained wassubjected to continuous feeding with a quantitative pump. Nitrogen gaswas continuously blown into the middle of the transport pipe, and thusthe oxygen concentration was adjusted to 0.5 [mg/L] or less.

Next, sodium persulfate/L-ascorbic acid were further continuously mixedby line mixing separately into the aqueous monomer solution (1) inamounts of 0.14 g/0.005 g (relative to 1 mole of the monomer),respectively. The mixture was supplied to a planar steel belt havingweirs at both ends to form a layer having a thickness of about 30 mm,and static aqueous solution polymerization (continuous beltpolymerization) was carried out continuously for 30 minutes at 95° C.(polymerization step).

A water-containing gel-like crosslinked polymer (1) (solid contentconcentration: 45 wt %) obtained by the polymerization step describedabove was finely divided to a size of about 1 mm by using a meat chopperhaving a hole diameter of 7 mm in an atmosphere at 60° C. (gel grainrefining (crushing) step). Subsequently, the crosslinked polymer wasloaded in a wide spread manner on a moving multi-hole plate of acontinuously ventilated band dryer (dew point of hot air: 30° C.) toform a layer having a thickness of 50 mm, and was dried for 30 minutesat 185° C. Subsequently, the crosslinked polymer was cooled by beingexposed to external air. Thus, a dry polymer (1) (solid contentconcentration: 96 wt %, temperature: 60° C.) was obtained (drying step).

The entire amount of the dry polymer (1) obtained was pulverized bycontinuously supplying the polymer to a three-stage roll mill (rollgaps: 1.0 mm/0.70 mm/0.50 mm from the top) (pulverization step), andthen was classified by using an oscillatory type circular sievingapparatus (frequency of vibration: 230 rpm, radial tilt (gradient): 11mm, tangential tilt (gradient): 11 mm, eccentricity: 35 mm, temperatureof the apparatus: 60° C., dew point of the atmosphere in the apparatus:13° C.) having a sieve diameter of 1,600 mm and metal sieve mesheshaving mesh opening sizes of 1,000 μm, 850 μm and 150 μm, respectively(material: SUS304, surface roughness of inner sieve surface Rz: 50 nm,surface roughness Ra: 4.8 nm, stretch tension: 50 [N/cm], area of sievemesh: 2 [m²/sheet]). The particle fraction between the 850-μm metalsieve mesh and the 150-μm metal sieve mesh was collected, and thus awater absorbent resin powder (1) in which the proportion of particleshaving a particle size of 850 μm to 150 μm was about 98 wt % (CRC: 36[g/g], solid content: 96 wt %, weight average particle size (D50): 450μm, σζ: 0.35) was obtained. Meanwhile, the temperature of the waterabsorbent resin powder obtained after the pulverization step andsupplied to the sieving apparatus, was maintained at 60° C. Furthermore,the stand on which this sieving apparatus was installed, was groundedwith a ground resistance value of 5Ω (removal of electricity). Further,the degree of pressure reduction inside the sieving apparatus wasadjusted to 0.11 kPa by an exhaust apparatus provided with a bag filter,and the sieving apparatus was ventilated with air having a dew point of10° C. and a temperature of 60° C., at a rate of 2 [m³/hr](classification step 1).

The water absorbent resin powder (1) obtained as described above wascontinuously supplied at a constant rate of 1,500 [kg/hr] to a highspeed continuous mixer (Turbulizer, 1,000 rpm), and a surface treatingagent solution consisting of a mixture liquid of 0.3 parts by weight of1,4-butanediol, 0.5 parts by weight of propylene glycol, and 2.7 partsby weight of pure water, relative to 100 parts by weight of the waterabsorbent resin powder, was mixed with the powder by spraying.Subsequently, the mixture obtained was heat treated continuously for 40minutes at 198° C. by using a paddle dryer (surface crosslinking step).Thereafter, the mixture was forcibly cooled to 60° C. by using the samepaddle dryer (cooling step).

Furthermore, the 850-μm pass powder was classified by using the sameoscillatory type circular sieving apparatus having a sieve diameter of1,600 mm as the apparatus used in the classification step 1 describedabove (temperature of the apparatus: 60° C.; mesh opening size: 850 μm;stretch tension of the mesh: 50 [N/cm]; material: SUS316; dew point ofthe atmosphere inside the apparatus: 12° C.). The particles remaining onthe sieve mesh having a mesh opening size of 850 μm were pulverizedagain, and then were mixed with the 850-μm pass powder. Thus, a particlesize-regulated water absorbent resin (1) (moisture content: 1.5 wt %,extractables: 8.7 wt %, weight average particle size (D50): 445 μm, σζ:0.39), in which the entire amount of the resin was a 850-μm pass powder,was obtained (particle size regulation step (classification step 2)).Meanwhile, the temperature of the water absorbent resin powder obtainedafter the pulverization step and supplied to the sieving apparatus, wasmaintained at 60° C. Furthermore, the stand on which this sievingapparatus was installed, was grounded (removal of electricity) with aground resistance value of 5Ω. Further, the degree of pressure reductioninside the sieving apparatus was adjusted to 0.11 kPa by an exhaustapparatus provided with a bag filter, and the sieving apparatus wasventilated with air having a dew point of 10° C. and a temperature of60° C., at a rate of 2 [m³/hr].

Subsequently, for the 850-μm pass powder obtained in the classificationstep 2 described above, fine powder having a particle size of less than120 μm was removed by using the same oscillatory type circular sievingapparatus having a sieve diameter of 1,600 mm as the apparatus used inthe classification step 1 described above (temperature of the apparatus:60° C.; mesh opening size: 120 μm; stretch tension of the mesh: 50[N/cm]; material: SUS316). Thus, a water absorbent resin (1) wasobtained (classification step 3). Meanwhile, the temperature of thewater absorbent resin powder obtained after the pulverization step andsupplied to the sieving apparatus, was maintained at 60° C. Furthermore,the stand on which this sieving apparatus was installed, was grounded(removal of electricity) with a ground resistance value of 5Ω. Further,the degree of pressure reduction inside the sieving apparatus wasadjusted to 0.11 kPa by an exhaust apparatus provided with a bag filter,and the sieving apparatus was ventilated with air having a dew point of10° C. and a temperature of 60° C., at a rate of 2 [m³/hr].

While the water absorbent resin (1) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.3 [g/g] (CRC), 24.7 [g/g] (AAP), and 42[×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. The details of the performance of the water absorbentresin (1) obtained are presented in Table 1. Meanwhile, even after alapse of 365 days (12 months), there was neither a deterioration ofphysical properties nor a change in the particle size, and the apparatusoperated stably.

Example 2

The same operation as that employed in Example 1 was carried out, exceptthat the stretch tension (tension) of the metal sieve mesh of theclassification step 3 used in Example 1 described above was changed to40 [N/cm], and thus a water absorbent resin (2) was obtained.

While the water absorbent resin (2) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.2 [g/g] (CRC), 24.6 [g/g] (AAP), and 38[×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. A slight decrease in the SFC was observed, but thedecrease was at a level without any problems, and the SFC value wasstabilized later. The details of the performance of the water absorbentresin (2) obtained are presented in Table 1. Meanwhile, even after alapse of 365 days (12 months), there was neither a deterioration ofphysical properties nor a change in the particle size, and the apparatusoperated stably.

Example 3

The same operation as that employed in Example 1 was carried out, exceptthat the stretch tension (tension) of the metal sieve mesh of theclassification step 3 used in Example 1 described above was changed to45 [N/cm], and thus a water absorbent resin (3) was obtained.

While the water absorbent resin (3) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.3 [g/g] (CRC), 24.7 [g/g] (AAP), and 40[×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. A slight decrease in the SFC was observed, but thedecrease was at a level without any problems, and the SFC value wasstabilized later. The details of the performance of the water absorbentresin (3) obtained are presented in Table 1. Meanwhile, even after alapse of 365 days (12 months), there was neither a deterioration ofphysical properties nor a change in the particle size, and the apparatusoperated stably.

Example 4

The same operation as that employed in Example 1 was carried out, exceptthat the stretch tension (tension) of the metal sieve mesh of theclassification step 3 used in Example 1 described above was changed to85 [N/cm], and thus a water absorbent resin (4) was obtained.

While the water absorbent resin (4) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.3 [g/g] (CRC), 24.7 [g/g] (AAP), and 42[×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. The details of the performance of the water absorbentresin (3) obtained are presented in Table 1. Meanwhile, since adeterioration of physical properties and a change in the particle sizewere confirmed after a stable operation of 180 days (6 months), thecauses were investigated. Since it was discovered that the metal sievemesh of the classification step 3 was partially destroyed, the metalsieve mesh was replaced.

Example 5

The same operation as that employed in Example 1 was carried out, exceptthat the temperature and the stretch tension (tension) of the metalsieve of the classification step 3 used in Example 1 described abovewere changed to 30° C. and 50 [N/cm], respectively, and thus a waterabsorbent resin (5) was obtained.

While the water absorbent resin (5) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.2 [g/g] (CRC), 24.6 [g/g] (AAP), and 38[×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. The details of the performance of the water absorbentresin (5) obtained are presented in Table 1. Meanwhile, althoughaggregates were observed in some of the water absorbent resin present onthe metal sieve (

“

”), even after a lapse of 365 days (12 months), there was neither adeterioration of physical properties nor a change in the particle size,and the apparatus operated stably.

Comparative Example 1

The same operation as that employed in Example 1 was carried out, exceptthat the stretch tension (tension) of all the metal sieve meshes of theclassification steps 1 to 3 used in Example 1 described above waschanged to 30 [N/cm], and thus a comparative water absorbent resin (1)was obtained.

While the comparative water absorbent resin (1) obtained wascontinuously produced, sampling was carried out for every 1 ton, and aperformance analysis was carried out for 20 tons of the water absorbentresin. The number of samples obtained was 20 samples, and the averageCRC, AAP and SFC values were 29.8 [g/g] (CRC), 23.9 [g/g] (AAP), and34[×10⁻⁷ cm³·s·g⁻¹] (SFC), respectively. The details of the performanceof the comparative water absorbent resin (1) obtained are presented inTable 1. Meanwhile, since a deterioration of physical properties and achange in the particle size were confirmed after a stable operation of60 days (2 months), the causes were investigated. Since it wasdiscovered that some of the metal sieve meshes of the classificationsteps were destroyed, the metal sieve meshes were replaced.

Comparative Example 2

The same operation as that employed in Example 1 was carried out, exceptthat the stretch tension (tension) of the metal sieve mesh of theclassification step 3 used in Example 1 described above was changed to30 [N/cm], and thus a comparative water absorbent resin (2) wasobtained.

While the comparative water absorbent resin (2) obtained wascontinuously produced, sampling was carried out for every 1 ton, and aperformance analysis was carried out for 20 tons of the water absorbentresin. The number of samples obtained was 20 samples, and the averageCRC, AAP and SFC values were 29.8 [g/g] (CRC), 24.7 [g/g] (AAP), and34[×10⁻⁷ cm³·s·g⁻¹] (SFC), respectively. The details of the performanceof the comparative water absorbent resin (2) obtained are presented inTable 1. Meanwhile, since a deterioration of physical properties and achange in the particle size were confirmed after a stable operation of90 days (3 months), the causes were investigated. Since it wasdiscovered that the metal sieve mesh of the classification step 3 waspartially destroyed, the metal sieve mesh was replaced.

Comparative Example 3

The same operation as that employed in Example 1 was carried out, exceptthat the stretch tension (tension) of the metal sieve mesh of theclassification step 3 used in Example 1 described above was changed to150 [N/cm], and thus a comparative water absorbent resin (3) wasobtained.

While the comparative water absorbent resin (3) obtained wascontinuously produced, sampling was carried out for every 1 ton, and aperformance analysis was carried out for 20 tons of the water absorbentresin. The number of samples obtained was 20 samples, and the averageCRC, AAP and SFC values were 30.3 [g/g] (CRC), 24.7 [g/g] (AAP), and42[×10⁻⁷ cm³·s·g⁻¹] (SFC), respectively. The details of the performanceof the comparative water absorbent resin (3) obtained are presented inTable 1. Meanwhile, since a deterioration of physical properties and achange in the particle size were confirmed after a stable operation of60 days (2 months), the causes were investigated. Since it wasdiscovered that the metal sieve mesh of the classification step 3 waspartially destroyed, the metal sieve mesh was replaced.

Comparative Example 4

The same operation as that employed in Example 1 was carried out, exceptthat the stretch tension (tension) of the metal sieve mesh of theclassification step 1 used in Example 1 describe above was changed to120 [N/cm], and thus a comparative water absorbent resin (4) wasobtained.

While the comparative water absorbent resin (4) obtained wascontinuously produced, sampling was carried out for every 1 ton, and aperformance analysis was carried out for 20 tons of the water absorbentresin. The number of samples obtained was 20 samples, and the averageCRC, AAP and SFC values were 30.1 [g/g] (CRC), 24.5 [g/g] (AAP), and41[×10⁻⁷ cm³·s·g⁻¹] (SFC), respectively. The details of the performanceof the comparative water absorbent resin (4) obtained are presented inTable 1. Meanwhile, since a deterioration of physical properties and achange in the particle size were confirmed after a stable operation of60 days (2 months), the causes were investigated. Since it wasdiscovered that some of the metal sieve mesh of the classification step1 were destroyed, the metal sieve mesh was replaced.

Example 6

The same operation as that employed in Example 1 was carried out, exceptthat an airbrush was installed below the metal sieve in theclassification step 3 of Example 1 described above, and classificationwas carried out by using air having a dew point of −30° C., and thus awater absorbent resin (6) was obtained.

While the water absorbent resin (6) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.3 [g/g] (CRC), 24.7 [g/g] (AAP), and 46 [×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. The details of the performance of the water absorbentresin (6) obtained are presented in Table 1.

Example 7

The same operation as that employed in Example 1 was carried out, exceptthat white (milky white) tapping balls having a diameter of 30 mm and apunching metal made of stainless steel (material: SUS304) having a holediameter of 20 mm were installed below the metal sieve in theclassification step 3 of Example 1 described above, and classificationwas carried out in the classification step 3 of Example 1 describedabove, and thus a water absorbent resin (7) was obtained.

While the water absorbent resin (7) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.4 [g/g] (CRC), 24.7 [g/g] (AAP), and 44[×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. The details of the performance of the water absorbentresin (7) obtained are presented in Table 1.

Example 8

The same operation as that employed in Example 1 was carried out, exceptthat classification was carried out after 1 wt % of calcium carbonatewas added as a classification aid particle to the metal sieve in theclassification step 3 of Example 1 described above, and a fine powder ofthe water absorbent resin and calcium carbonate were removed together,and thus a water absorbent resin (8) was obtained.

While the water absorbent resin (8) obtained was continuously produced,sampling was carried out for every 1 ton, and a performance analysis wascarried out for 20 tons of the water absorbent resin. The number ofsamples obtained was 20 samples, and the average CRC, AAP and SFC valueswere 30.4 [g/g] (CRC), 24.1 [g/g] (AAP), and 45 [×10⁻⁷ cm³·s·g⁻¹] (SFC),respectively. The details of the performance of the water absorbentresin (8) obtained are presented in Table 1.

TABLE 1 Sieve Decrement Tension [N/cm] Classifi- replace- SFC of SFCClassifi- Classifi- Classifi- cation ment CRC AAP [×10⁻⁷ [×10⁻⁷ cation 1cation 2 cation 3 step 3 [days] [g/g] [g/g] cm³ · s · g⁻¹] cm³ · s ·g⁻¹] Example 1 50 50 50 — 365 30.3 24.7 42 2 Example 2 50 50 40 — 36530.2 24.6 38 3 Example 3 50 50 45 — 365 30.3 24.7 40 2 Example 4 50 5085 — 180 30.3 24.7 42 2 Example 5 50 50 50 — 365 30.2 24.6 38 3Comparative 30 30 30 — 60 29.8 23.9 34 5 Example 1 Comparative 50 50 30— 90 29.8 24.7 34 5 Example 2 Comparative 50 50 150  — 60 30.3 24.7 42 6Example 3 Comparative 120  50 50 — 60 30.1 24.5 41 6 Example 4 Example 650 50 50 O1) 30.3 24.7 46 1 Example 7 50 50 50 O2) 30.4 24.7 44 1Example 8 50 50 50 O3) 30.3 24.1 45 1 1) Airbrush, 2) Tapping material,or 3) Calcium carbonate, was respectively used.

CONCLUSIONS

As indicated in Table 1, by controlling the tension (stretch tension) ofthe metal sieve meshes 35 to 100 [N/cm], and/or by using a tappingmaterial, an airbrush and a classification aid, the various physicalproperties (SFC and the like) of the water absorbent resins obtainableare enhanced. Furthermore, by controlling the stretch tension (tension)of the metal sieve meshes to 35 to 100 [N/cm] in all the classificationsteps, the time period of continuous operation, that is, the time forreplacement of metal sieve meshes, is increased. Further, it is shownthat the stretch tension (tension) of the metal sieve meshes affects thephysical property of a water absorbent resin, particularly the physicalproperty obtained after surface crosslinking (particularly, liquidpermeability); however, it is understood that when the stretch tension(tension) of the metal sieve meshes is controlled to 35 to 100 [N/cm],the metal sieve meshes stably operate for a long time period, and thestable operation contributes to stabilization of the physical propertyeven at the time of continuous operation. Furthermore, generation orincorporation of aggregates can be reduced by increasing the temperatureof the sieves. Patent Literature 1 to 30 described above neitherdiscloses nor suggests the idea of the present invention.

INDUSTRIAL APPLICABILITY

A water absorbent resin which is free from coloration or foreignmaterials can be stably produced at low cost by continuous production ina huge scale (particularly 1 [t/hr] or more), and therefore the waterabsorbent resin of the present invention can be used in various sanitarymaterials including paper diapers and sanitary napkins, as well as invarious applications of water absorbent resins.

In addition, the present patent application is based on Japanese PatentApplication Nos. 2010-061223 and 2010-061224 filed Mar. 17, 2010, theentire disclosure of which is incorporated herein by reference.

The invention claimed is:
 1. A method for producing a water absorbentresin comprising: a polymerization step of polymerizing an aqueoussolution of acrylic acid (salt) to obtain a water-containing gel-likecrosslinked polymer; a drying step of drying the water-containinggel-like crosslinked polymer to obtain a water absorbent resin powder; aclassification step of classifying the water absorbent resin powder; anda surface crosslinking step of surface crosslinking the water absorbentresin powder, wherein in the classification step that is carried beforethe surface crosslinking step and/or after the surface crosslinkingstep, stretch tension (tension) of a metal sieve mesh used in theclassification step is from 35 [N/cm] to 100 [N/cm], the methodsatisfying one or more of the following three requirements: (1) anairbrush is installed below the metal sieve mesh used in theclassification step, in which the airbrush uses dry air having a dewpoint of 0° C. or lower, and the classifying apparatus is heated at 40°C. to 80° C.; (2) in the classification step, a classification aidparticle having specific gravity larger than that of the water absorbentresin powder is added, and a fine powder of the water absorbent resinand the classification aid particle are removed, in which theclassification aid particle is an inorganic fine particle or an organicfine particle; and (3) plural tapping balls or tapping blocks areinstalled below the metal sieve mesh used in the classification step, inwhich a gas stream passes through in the classification step, and thedew point of the gas stream is 15° C. or lower.
 2. The method accordingto claim 1, wherein the material of the metal sieve mesh is SUS304 orSUS316, and the surface roughness (Rz) of the inner surface of the metalsieve mesh is 800 nm or less.
 3. The method according to claim 1,wherein requirement (2) or (3) is satisfied and the metal sieve mesh isheated or kept warm to a temperature of 40° C. or higher.
 4. The methodaccording to claim 1, wherein requirement (2) is satisfied, a gas streampasses through in the classification step, and the dew point of the gasstream is 15° C. or lower.
 5. The method according to claim 1, whereinin the classification step, removal of electricity is carried out by aground connection with a ground resistance value of 100Ω or less.
 6. Themethod according to claim 1, wherein the classification step is carriedout under reduced pressure.
 7. The method according to claim 1, whereinthe damage to the metal sieve mesh is monitored by an acoustic emission(AE) method.